Cambodian – Vietnamese War The Cambodian – Vietnamese War was an armed conflict between the Socialist Republic of Vietnam and Democratic Kampuchea . The war began with isolated clashes along the land and maritime boundaries of Vietnam and Kampuchea between 1975 and 1977 , occasionally involving division - sized military formations . On 25 December 1978 , Vietnam launched a full - scale invasion of Kampuchea and subsequently occupied the country and removed the Khmer Rouge from power . During the Vietnam War , Vietnamese and Khmer Rouge communists had formed an alliance to fight U . S . - backed regimes in their respective countries . Despite their open display of cooperation with the Vietnamese , the Khmer Rouge leadership feared that the Vietnamese communists were scheming to form an Indochinese federation with Vietnam as the dominant force in the region . In order to preempt an attempt by the Vietnamese to dominate them , the Khmer Rouge leadership began purging Vietnamese - trained personnel within their own ranks as the Lon Nol regime capitulated in 1975 . Then , in May 1975 , the newly formed Democratic Kampuchea , dominated by the Khmer Rouge , began waging a war against Vietnam , which was marked by an attack on the Vietnamese island of Phú Quốc . In spite of the fighting that had occurred between the two countries , the leaders of reunified Vietnam and Kampuchea made several public diplomatic exchanges throughout 1976 to highlight the supposedly strong relations between them . However , behind the scenes , Kampuchean leaders continued to fear what they perceived as Vietnamese expansionism . As such , on 30 April 1977 , they launched another major military attack on Vietnam . Shocked by the Kampuchean assault , Vietnam launched a retaliatory strike at the end of 1977 in an attempt to force the Kampuchean Government to negotiate . In January 1978 , the Vietnamese military withdrew because their political objectives had not been achieved . Small - scale fighting continued between the two countries throughout 1978 , as China tried to mediate peace talks between the two sides . However , neither country could reach an acceptable compromise at the negotiation table . By the end of 1978 , Vietnamese leaders decided to remove the Khmer Rouge - dominated regime of Democratic Kampuchea , perceiving it as being pro - Chinese and too hostile towards Vietnam . On 25 December 1978 , 150 , 000 Vietnamese troops invaded Democratic Kampuchea and overran the Kampuchean Revolutionary Army in just two weeks . On 8 January 1979 , a pro - Vietnamese People ' s Republic of Kampuchea ( PRK ) was established in Phnom Penh , marking the beginning of a ten - year Vietnamese occupation . During that period , the Khmer Rouge ' s Democratic Kampuchea continued to be recognised by the United Nations as the legitimate government of Kampuchea , as several armed resistance groups were formed to fight the Vietnamese occupation . Behind the scenes , Prime Minister Hun Sen of the PRK regime approached factions of the Coalition Government of Democratic Kampuchea ( CGDK ) to begin peace talks . Under heavy diplomatic and economic pressure from the international community , the Vietnamese Government implemented a series of economic and foreign policy reforms , which led to their withdrawal from Kampuchea in September 1989 . At the Third Jakarta Informal Meeting in 1990 , under the Australian - sponsored Cambodian Peace Plan , representatives of the CGDK and the PRK agreed to a power - sharing arrangement by forming a unity government known as the Supreme National Council ( SNC ) . The SNC ' s role was to represent Cambodian sovereignty on the international stage , while the United Nations Transitional Authority in Cambodia ( UNTAC ) was tasked with supervising the country ' s domestic policies until a Cambodian government was elected by the people through a peaceful , democratic process . Cambodia ' s pathway to peace proved to be difficult , as Khmer Rouge leaders decided not to participate in the general elections , but instead they chose to disrupt the electoral process by launching military attacks on UN peacekeepers and killing ethnic Vietnamese migrants . In May 1993 , Sihanouk ' s FUNCINPEC movement defeated the Cambodian People ' s Party ( CPP ) , formerly the Kampuchean People ' s Revolutionary Party ( KPRP ) , to win the general elections . However , the CPP leadership refused to accept defeat and they announced that the eastern provinces of Cambodia , where most of the CPP ' s votes were drawn from , would secede from Cambodia . To avoid such an outcome , Norodom Ranariddh , the leader of FUNCINPEC agreed to form a coalition government with the CPP . Shortly afterwards , the constitutional monarchy was restored and the Khmer Rouge was outlawed by the newly formed Cambodian Government . Background . Cambodian - Vietnamese history . Angkor , the seat of the Khmer Empire was subjected to Vietnamese influence as early as the 13th century . Vietnamese influence spread gradually and indirectly , and it was not until the early 19th century that Vietnam exercised direct control . In 1813 , Nak Ong Chan gained the Cambodian throne with the help of Vietnam , and under his rule Cambodia became a protectorate . Following his death in 1834 , Vietnam colonised Cambodia ; it was governed under a Vietnamese administration and termed a Vietnamese ‘ province ’ . Throughout the 1830s , Vietnam attempted to erase Khmer culture , which had derived the basis of Cambodian society , dress and religion from India rather than China . The trend of Vietnamese dominance continued during French colonization , under which Cambodia was forced to cede much of its southern cone ( would later be Saigon , the Mekong Delta and Tây Ninh ) to the Vietnamese . The Khmer Rouge later justified their incursions into Vietnam as an attempt to regain the territories which Cambodia lost during the previous centuries . Rise of communism . The communist movement in Cambodia and Vietnam began before World War II with the founding of the Indochinese Communist Party ( ICP ) , almost exclusively dominated by the Vietnamese , originally meant to fight French colonial rule in Indochina . In 1941 , Nguyen Ai Quoc ( commonly known by his alias Ho Chi Minh ) founded the Viet Nam Doc Lap Dong Minh Hoi , or the Viet Minh . When the Japanese were defeated at the end of World War II , he initiated the first Indochinese war of independence against the French . During this time , Vietnamese forces made extensive use of Cambodian territory to transport weapons , supplies , and troops . This relationship lasted throughout the Vietnam War , when Vietnamese communists used Cambodia as a transport route and staging area for attacks on South Vietnam . In 1951 , Vietnam guided the establishment of a separate Cambodian communist party , the Kampuchean People ' s Revolutionary Party ( KPRP ) , which allied with a nationalist separatist Cambodian movement , the Khmer Serei ( Free Khmers ) , in order to pursue independence . In accordance with the 1954 Geneva Accords negotiating the end of the French domination , newly created communist North Vietnam pulled all of its Viet Minh soldiers and cadres out of Cambodia ; however , since the KPRP was staffed primarily by ethnic Vietnamese or Cambodians under its tutelage , approximately 5 , 000 Communist cadres went with them . The power vacuum the Vietnamese communists left in its wake in Cambodia was soon filled by the return of a young group of Cambodian communist revolutionaries , many of whom received their communist education in France . In 1960 , the KPRP changed its name to the Kampuchean Communist Party ( KCP ) , and the name was later adopted by the majority coalition that formed around Saloth Sar ( Pol Pot ) , Ieng Sary and Khieu Samphan as the ‘ true ’ political institution memorialising the KCP . This clique became the genesis of the Khmer Rouge , and its doctrine was heavily influenced by the Maoist ideology . Democratic Kampuchea and the Khmer Rouge . The Khmer Rouge government adopted the mysterious term Angkar , or " the organisation " and , until 1977 , the identities of its leaders remained confidential . The official head of state was Khieu Samphan , but the two men in control of the Party were Pol Pot and Ieng Sary . The ultimate objective of the Khmer Rouge was to erase the structure of the Cambodian state , which they viewed as feudal , capitalist , and oriented around the agendas of both the landholding elite and imperialists . In its place , they hoped to create a classless society based entirely on worker - peasants . The radical ideologies and goals of the Khmer Rouge were alien concepts to the masses . In fact , the socialist revolution held very little popular appeal , which led Pol Pot and his cadres to use ultra - nationalist sentiment , repressive and murderous rule , and propaganda aimed at demonising the Vietnamese to maintain their tenuous control . During the five years of the Khmer Rouge rebellion from 1970 to 1975 , the support of North Vietnam , in conjunction with China , was essential to its eventual triumph . However , even before the Vietnam War ended , the relationship between the Khmer Rouge — which was in the process of seizing power from a US - backed regime headed by Lon Nol — and Vietnam was strained . Clashes between Vietnamese communists and Khmer Rouge forces began as early as 1974 , and the following year Pol Pot signed a treaty codifying the ‘ friendship ’ between the Khmer Rouge and China . Diplomacy and military action . 1975 – 76 , from fighting to building friendly relations . The conclusion of the Indochina conflict in April 1975 immediately brought a new conflict between Vietnam and Kampuchea . Although both the North Vietnamese and the Khmer Rouge had previously fought side by side , the leaders of the newly created Democratic Kampuchea continued to view North Vietnam with great suspicion , because they believed the Vietnamese Communists had never given up their dream of creating an Indochinese federation with Vietnam as the leader . For that reason , the Kampuchean Government decided to remove all North Vietnamese military forces from Kampuchean territory shortly after their capture of Phnom Penh in 1975 . In the first major clash between the two former allies , the Kampuchean Revolutionary Army invaded the Vietnamese island of Phú Quốc on 1 May 1975 , which they claimed was part of Kampuchea ’ s territory . Nine days later , on 10 May 1975 , the Kampuchean military continued its incursion by capturing Thổ Chu , where it executed 500 Vietnamese civilians . The Vietnamese military immediately responded to Kampuchean actions by launching a counter - attack and removing Kampuchean forces from Phu Quoc and Tho Chu , and then invading the Kampuchean island of Koh Wai . In June 1975 , while on a visit to Hanoi , Kampuchean leader Pol Pot proposed that Vietnam and his country should sign a treaty of friendship and begin discussions on border disputes . However , those discussions never materialised , and the Kampucheans claimed that Vietnam turned down both offers . In August 1975 , Vietnam returned the island of Koh Wai to Kampuchea and formally recognised Kampuchean sovereignty over the island . Following those incidents , both countries attempted to improve their diplomatic relations with a series of congratulatory messages and exchange visits . On 17 April 1976 , Vietnamese leaders sent a message to congratulate Khieu Samphan , Nuon Chea and Pol Pot on their " elections " as President , President of the People ’ s Representatives and Premier of Kampuchea , respectively . Furthermore , the Vietnamese even denounced the alleged " U . S . bombing " of Siem Reap in February 1976 , thereby reinforcing the Kampuchean ’ s fictitious claim over the incident . In response , in June 1976 , the Kampuchean leadership sent a message to the Provisional Revolutionary Government of the Republic of South Vietnam , which had governed South Vietnam since the fall of the U . S . - backed regime , congratulating them on the seventh anniversary of their establishment . In July 1976 , following the establishment of the Socialist Republic of Vietnam as a reunified country , Phnom Penh Radio broadcast a commentary which proclaimed the " militant solidarity and friendship between peoples of Democratic Kampuchea and the Socialist Republic of Vietnam grow constantly greener and sturdier " . However , during that same month , Premier Pol Pot publicly hinted at tensions between Vietnam and Kampuchea when he told a visiting Vietnamese media delegation that there were “ obstacles and difficulties ” in the relationship between the two countries . Nonetheless , on 21 September 1976 , the first air service connecting Hanoi and Ho Chi Minh City with Phnom Penh was established . Then in December 1976 , the Kampuchean Revolutionary Organisation sent greetings to the Vietnamese Communist Party during their Fourth Congress . 1977 , build - up to war . Towards the end of 1976 , while Vietnam and Kampuchea publicly appeared to be improving their relationships , the private suspicions of both countries ' leadership grew . From the Vietnamese perspective , they were the patron of genuine Marxist – Leninist revolutions in South East Asia , so it was vital for them to exercise control over the Kampucheans and the Laotians . Indeed , that was the reason North Vietnam supported the Khmer Rouge during their fight against the Lon Nol regime , in the hope that the Kampuchean communists would adopt a pro - Vietnamese line upon their victory in the same way the Pathet Lao had done . However , their hopes were dashed as early as 1973 , because North Vietnamese military formations operating in Khmer Rouge - occupied territories were occasionally subjected to armed attacks by their own allies . The Vietnamese position inside Kampuchea was further weakened after the end of the war , as there were no pro - Vietnamese elements left within the Kampuchean Communist Party . Thus , when the pro - Chinese Pol Pot and his brother - in - law Ieng Sary resigned from their respective positions as Premier and Foreign Minister in September 1976 , Vietnamese Prime Minister Phạm Văn Đồng and General Secretary of the Communist Party Lê Duẩn were optimistic that Vietnam could exercise greater influence on the Kampucheans . In a private meeting with the Soviet ambassador to Vietnam on 16 November 1976 , Lê Duẩn dismissed both Ieng Sary and Pol Pot as " bad people " for their pro - Chinese policies . Le Duan then asserted that Nuon Chea , who had ascended to the position of Premier of Democratic Kampuchea as Pol Pot ' s replacement , was a person of pro - Vietnamese orientation , so Vietnam could exercise its influence through him . However , the events which developed over the next few months would prove Lê Duẩn had been mistaken in his assessment of Nuon Chea . Meanwhile , in Phnom Penh , the Kampuchean leadership had developed a seething fear and hatred of the Vietnamese leadership as a result of Vietnam ’ s historical dominance over their country . From the Kampuchean perspective , the Vietnamese strategy to dominate Indochina involved infiltrating the communist parties of Kampuchea and Laos with Vietnamese - trained cadres . For that reason , when the first group of North Vietnamese - trained Khmer Rouge personnel returned to the country , they were immediately purged from the Party . During the months following the defeat of the Lon Nol regime , Pol Pot continued to purge his own party and the Government of Democratic Kampuchea of those who he believed to be Soviet and Vietnamese agents . Then , in the context of the triumphalism , which prevailed over the Khmer Rouge leadership in a war that they claimed they had single - handedly defeated the " American imperialist " , Democratic Kampuchea began to embark on a war against Vietnam . As the Kampuchean Revolutionary Army made preparations for its war against Vietnam , state - controlled media in Vietnam sent congratulatory messages to the Government of the Democratic Kampuchea on the second anniversary of its establishment , on 17 April 1977 . On 30 April 1977 , the second anniversary of the fall of Saigon , the Kampuchean reply came in the form of a military attack against the Vietnamese provinces of An Giang and Châu Đốc , killing hundreds of Vietnamese civilians . The Vietnam People ' s Army responded by moving its troops to areas attacked by Kampuchea and , on 7 June 1977 , Vietnam proposed high - level talks to discuss outstanding issues . On 18 June 1977 , the Kampuchean Government replied by demanding that Vietnam remove all of its military units from the disputed areas , and create a demilitarised zone between the opposing forces . Both sides ignored each other ' s proposals , and the Kampuchean Revolutionary Army continued sending soldiers across the border to attack Vietnamese towns and villages . In September 1977 , Kampuchean artillery struck several Vietnamese villages along the border , and six villages in Đồng Tháp Province were overrun by Kampuchean infantry . Shortly afterwards , six divisions of the Kampuchea Revolutionary Army advanced about into Tay Ninh Province , where they killed more than 1 , 000 Vietnamese civilians . Angered by the scale of Kampuchean assaults , the Vietnam People ’ s Army assembled eight divisions , estimated at around 60 , 000 soldiers , to launch a retaliatory strike against Kampuchea . On 16 December 1977 , Vietnamese divisions , with support from elements of the Vietnam People ' s Air Force , crossed the border along several axes with the objective of forcing the Kampuchean Government to negotiate . On the battlefield , the Kampucheans quickly lost ground as their combat units were pushed back by the Vietnamese . By the end of December 1977 , Vietnam had won a clear military victory over Kampuchea , as Vietnamese formations marched through Svay Rieng Province and only stopped short of entering the provincial capital . Despite the ferocity of the Vietnamese retaliation , the Kampuchean Government remained defiant . On 31 December 1977 , Khieu Sampham declared that the Kampuchean Government would " temporarily " sever diplomatic relations with Vietnam until the Vietnamese military withdraw from the " sacred territory of Democratic Kampuchea " . On 6 January 1978 , Vietnamese divisions were only from Phnom Penh , but the Vietnamese Government decided to withdraw its forces from Kampuchea because they had failed to achieve Vietnam ’ s political objective . During the withdrawal , the Vietnamese military also evacuated thousands of prisoners and civilian refugees , including future leader Hun Sen . 1978 , preparations for regime change . Instead of being sobered by the Vietnamese show of force , the Kampuchean Government boasted that the Vietnamese withdrawal was a major victory for Democratic Kampuchea , comparing it to the “ defeat of U . S . imperialism ” on 17 April 1975 . The Kampucheans went on further to proclaim that " our 6 January victory over the annexationist , Vietnamese aggressor enemy has given all of us greater confidence in the forces of our people and nation , in our Kampuchean Communist Party and our Kampuchean Revolutionary Army , and in our Party ’ s line of people ’ s war " . The Kampuchean leadership claimed that one Kampuchean soldier was equal to 30 Vietnamese soldiers , so if Kampuchea could raise two million soldiers from a population of eight million , it could wipe out Vietnam ’ s population of 50 million and still have six million people left . In reality , Kampuchean leaders simply ignored the condition of the population in their own country and Vietnam ; the Vietnamese , though poor , were in good physical condition , while Kampuchea ' s population was physically and mentally exhausted from years of hard labour , starvation and disease . In addition to the disparity in population , there was also a great disparity between the fighting capabilities of the armed forces of the two countries . In 1977 , Vietnam was estimated to have 615 , 000 soldiers and 900 tanks , supported by a 12 , 000 - member air force with 300 combat aircraft , including one squadron of light bombers . In comparison , Kampuchea had an army of 70 , 000 , only a few heavy tanks , 200 armoured vehicles , and limited air capability . Despite facing such heavy odds , Kampuchea showed no signs of hesitation as its military continued to assault Vietnam ’ s border regions . In January 1978 , Kampuchean forces still held portions of Vietnamese territory and began overrunning Vietnamese outposts in Hà Tiên Province . On 27 January 1978 , Vietnam started calling on the Kampuchean military along the border regions to overthrow the Khmer Rouge regime . Against the backdrop of military clashes , between 9 January and 20 February 1978 , Vietnamese Deputy Foreign Minister Phan Hien made several trips to Beijing to hold discussions with representatives of the Kampuchean Government , which ultimately proved to be fruitless . On 18 January 1978 , China attempted to mediate further negotiations between Kampuchea and Vietnam when Vice Premier Deng Yingchao travelled to Phnom Penh , where she was met with strong resistance by Kampuchean leaders . Meanwhile , Vietnamese government officials began conducting secret meetings with So Phim , the Khmer Rouge leader in Kampuchea ' s Eastern Military Zone , to plan a military uprising backed by Vietnam . During that same period , military setbacks experienced by the Kampuchean Revolutionary Army in the Eastern Military Zone prompted Pol Pot to label the region as a " nest of traitors " . In order to purge the Eastern Military Zone of those he perceived to have been contaminated by the Vietnamese , Pol Pot ordered military units from the Southwest Zone to move into eastern Kampuchea and eliminate the " hidden traitors " . Unable to withstand an attack from the Kampuchea Government , So Phim committed suicide while his deputy Heng Samrin defected to Vietnam . On 12 April 1978 , the Kampuchean Government declared they and Vietnam could negotiate again if the Vietnamese gave up their expansionist ambitions and recognised Kampuchea ’ s sovereignty . However , there was also a pre - condition requiring Vietnam to meet several obligations through a seven - month trial ceasefire . The Vietnamese Government immediately rejected the demand and , in response , two Kampuchean divisions penetrated up to into Vietnamese territory , and massacred over 3 , 000 Vietnamese civilians in the village of Ba Chúc in An Giang Province . In June 1978 , the Vietnamese Air Force started bombing Kampuchean positions along the border regions , flying about 30 bombing sorties per day and inflicting heavy casualties on the Kampucheans . By that stage in the conflict , most surviving leaders of the Eastern Military Zone had escaped into Vietnam , where they assembled at various secret camps with the purpose of forming a Vietnamese - backed " liberation army " to fight against Pol Pot ' s Khmer Rouge regime . Meanwhile , the Vietnamese Communist Party Politburo was meeting in Hanoi to discuss its strategy for Kampuchea . It concluded that the Khmer Rouge regime was a proxy of China , which had been trying to fill the power vacuum following the withdrawal of the United States . As such , China was identified as Vietnam ’ s main enemy , and its client regime in Phnom Penh had to be removed by conventional military force , because the Vietnamese adaptation of the Maoist " people ' s war " doctrine had not been a success against the Khmer Rouge ’ s security apparatus . To reflect the attitude of the country ’ s leaders , Vietnam ' s state - controlled media stepped up its propaganda war against the Khmer Rouge , with the Vietnamese Communist " Nhân Dân " newspaper regularly calling for international intervention to save the Kampuchean people from domestic terror initiated by the Khmer Rouge regime . Furthermore , instead of sending congratulatory messages like they had done in the previous years , the Vietnamese media changed their tone and began referring to the Kampuchean Government as the " Pol Pot - Ieng Sary clique " as the Kampuchean military continued their campaign in Vietnam . By the end of June , the Vietnamese military assembled a multi - division task force to launch another limited - objective campaign against the Kampucheans . Again , the Vietnamese pushed the Kampuchean forces back into the provincial cities of Suong and Prey Veng and then pulled out . However , as they had done before , the Kampuchean Army moved its artillery towards the border and continued shelling Vietnamese villages as though nothing had happened . During the second half of 1978 , Vietnamese leaders devoted much of their energy towards the military campaign against the Khmer Rouge regime , by seeking political support from the Soviet Union . In a briefing with Vietnamese Foreign Ministry officials on 25 July 1978 , the Soviet chargé d ' affaires in Hanoi was told that the Kampuchean Government had deployed 14 of its 17 regular army divisions and 16 local regiments along the border with Vietnam . Then , in early September 1978 , Lê Duẩn informed the Soviet ambassador that Vietnam aimed to " solve fully this question of Kampuchea by the beginning of 1979 " . While Vietnam was laying the political foundation for the military campaign against Kampuchea , Soviet ships were reported to be unloading military hardware and ammunition in Cam Ranh Bay . In October 1978 , Vietnamese radio broadcast what they claimed were accounts of uprisings against the Khmer Rouge regime , urging members of the Kampuchean military either to overthrow the " Pol Pot - Ieng Sary clique " or defect to Vietnam . In a major turning point in the course of Soviet - Vietnamese and Sino - Vietnamese diplomatic relations , and ultimately the Vietnamese invasion of Kampuchea , a Treaty of Friendship and Cooperation was signed between Vietnam and the Soviet Union on 3 November 1978 , which guaranteed the former of vital Soviet military aid in the scenario that China intervened in the conflict . Later , in November 1978 , a command and control headquarters was established for the planned invasion of Kampuchea , with Senior General Lê Đức Anh taking full control of Vietnamese military units along the border areas . The Vietnamese government drafted 350 , 000 men into the military to replace earlier losses and augment its units along the border . While the new recruits were completing training , ten divisions were deployed to the border regions of Long An , Đồng Tháp and Tây Ninh provinces . Vietnam also shifted three divisions based in Laos south towards the Laos - Kampuchea border . On 13 December 1978 , the Chinese Government warned Vietnam that its patience was limited , and that Vietnam would be punished if it behaved in an " unbridled fashion " . Nonetheless , the final piece of the Vietnamese strategy emerged when Vietnam announced the formation of the Kampuchean United Front for National Salvation ( KUFNS ) in the " liberation zones " of Kampuchea . Hanoi claimed that KUFNS was an independent Kampuchean communist movement , with members drawn from all walks of life . Heng Samrin , formerly a member of the Khmer Rouge and commander of the Kampuchean 4th Division , was the chairman of the KUFNS Central Committee . Previously , the KUFNS was known as the Provisional Revolutionary Government of Kampuchea ( PRGK ) , which consisted of 300 former Khmer Rouge cadres who defected to Vietnam . The PRGK regularly sent representatives abroad in search of support , before Vietnam abandoned the ‘ people ’ s war ’ concept in favour of a conventional military campaign , similar to the Soviet invasion of Czechoslovakia in 1968 . Not to be outdone by the Vietnamese military build - up , the Government of Democratic Kampuchea was busy strengthening its armed forces with Chinese support . In previous years , China had only provided the Kampuchean Revolutionary Army with a limited amount of arms and ammunition , but as relations with Vietnam worsened in 1978 , Beijing established additional supply routes through Kampuchea and increased the volume of military hardware which travelled down each route . On the eve of the Vietnamese invasion , Kampuchea had an estimated 73 , 000 soldiers in the Eastern Military Zone bordering Vietnam . At that time , all branches of the Kampuchean armed forces were significantly strengthened by large quantities of Chinese - made military equipment , which included fighter aircraft , patrol boats , heavy artillery , anti - aircraft guns , trucks and tanks . Additionally , there were between 10 , 000 and 20 , 000 Chinese advisers in both military and civilian capacities , providing their support to the Khmer Rouge regime . Invasion of Kampuchea . Early in December 1978 , Kampuchea ’ s new found strength was tested when a Vietnamese offensive , consisting of two divisions , crossed the border and moved towards the town of Kratie , while other support divisions were deployed along local routes to cut off the logistical tail of Kampuchean units . Despite enjoying generous support from China , the Kampuchean military could not withstand the Vietnamese offensive and suffered heavy casualties . Finally , on 25 December 1978 , Vietnam launched a full - scale invasion using 13 divisions , estimated at 150 , 000 soldiers well - supported by heavy artillery and air power . Initially , Kampuchea directly challenged Vietnam ’ s military might through conventional fighting methods , but this tactic resulted in the loss of half of the Kampuchean Revolutionary Army within two weeks . Heavy defeats on the battlefield prompted much of the Kampuchean leadership to evacuate towards the western region of the country . On 7 January 1979 , the Vietnamese Army entered Phnom Penh along with members of the KUFNS . On the following day , a pro - Vietnamese Kampuchean state , known as the People ' s Republic of Kampuchea ( PRK ) , was established , with Heng Samrin as the Chief of State and Pen Sovan as General Secretary of the Kampuchean People ' s Revolutionary Party . The Khmer Rouge leadership , with much of its political and military structures shattered by the Vietnamese invasion , was forced to take refuge in Thailand , where it was warmly welcomed by the Thai Government . Despite the overwhelming economic challenges brought by the Khmer Rouge and the accompanying refugees , the Thai Government sheltered and protected the Khmer Rouge at Khao Larn camp in Trat Province . Meanwhile , in Phnom Penh , the new Kampuchean regime tried to rebuild the country ’ s economic and social life , which was largely destroyed by decades of political upheavals and constant warfare . However , the new Kampuchean Government ’ s effort to rebuild the country was severely hampered by the lack of educated and qualified personnel , as most educated people had either fled the country or had been murdered by the Khmer Rouge regime during the previous four years . By the end of the year , the new regime ’ s attempts at nation - building were further challenged by several anti - Vietnamese resistance groups operating along the western regions of the country . Response from the international community . Shortly after the fall of Phnom Penh to Vietnamese forces and their Kampuchean allies in January 1979 , representatives of Democratic Kampuchea called for an emergency meeting of the United Nations Security Council , so Prince Norodom Sihanouk could present the deposed regime ' s case . Despite strong objections from the Soviet Union and Czechoslovakia , the UN Security Council gave Sihanouk this chance . Although Sihanouk distanced himself from the human rights abuses of the Khmer Rouge , he accused Vietnam of using aggression to violate Kampuchea ’ s sovereignty . As such , he demanded all UN countries to suspend aid to Vietnam and not recognise the Vietnamese - installed regime . Subsequently , seven non - aligned members of the UN Security Council submitted a draft resolution calling for a ceasefire and the withdrawal of all foreign forces from Kampuchea , which was endorsed by China , France , Norway , Portugal , the United States and the United Kingdom . However , the resolution was not approved due to strong opposition from the Soviet Union and Czechoslovakia . Critics of Vietnamese actions held that they did not invade Cambodia out of any noble desire to stop the atrocities committed by Pol Pot ' s regime but rather to consolidate their domination of Indochina . Between 16 – 19 February 1979 Vietnam and the new Kampuchean regime held a summit meeting which concluded with the two countries signing the Treaty of Peace , Friendship and Cooperation . Article 2 of the treaty stated that the security of Vietnam and Kampuchea were interrelated ; thus they would help defend each other “ against schemes and acts of sabotage by the imperialist and international reactionary forces ” , thereby legitimising the presence of Vietnamese troops on Kampuchean soil . Soon afterwards , the Soviet Union , the socialist countries of Eastern Europe and India immediately recognised the Vietnamese - installed People ’ s Republic of Kampuchea . The Soviet Government praised the PRK ' s " remarkable victory " and expressed its full support for the regime ' s advance towards socialism . Furthermore , the Soviets harshly criticised the Khmer Rouge regime ' s record of terror , which they implied had been imposed by China . At the 34th Session of the UN General Assembly , representatives of the People ' s Republic of Kampuchea and Democratic Kampuchea both claimed the right to represent their country . The former also notified the member nations of the UN Security Council that it was the sole legitimate representative of Kampuchea and its people . In response , the UN Credentials Committee decided to recognise Democratic Kampuchea by a vote of six to three , despite the Khmer Rouge ' s blood - stained record while in power . Accordingly , representatives of Democratic Kampuchea were allowed to be seated in the General Assembly , with strong support from China . By January 1980 , 29 countries had established diplomatic relations with the People Republic of Kampuchea , yet nearly 80 countries still recognised the legitimacy of the deposed Democratic Kampuchea . At the same time , the Western powers and the member countries of the Association of South East Asian Nations ( ASEAN ) also voiced strong condemnation of Vietnam ’ s use of force to remove the Khmer Rouge regime . Thailand , which shares an border with Kampuchea and has historically feared Vietnam ' s expansionism , demanded that Vietnam immediately remove its troops from Kampuchea so its people could elect a government free from foreign intervention . Indonesia , Malaysia , the Philippines and Singapore showed their support for Thailand ' s position . Furthermore , ASEAN viewed Vietnam ’ s invasion and subsequent occupation of Kampuchea , which received strong Soviet support , as an intolerable threat to the region ’ s security and stability . That view was shared by China , which went as far as accusing Vietnam of forcing Kampuchea into an Indochinese federation to serve as an outpost of Soviet global hegemony . The United States , which never maintained any form of diplomatic ties with the Khmer Rouge ’ s Democratic Kampuchea , showed strong support for the membership of their former enemy in the UN General Assembly , and echoed ASEAN ' s call for an immediate withdrawal of Vietnamese military forces from Kampuchea . China invades Vietnam . In February 1979 , China retaliated against Vietnam ' s invasion of Kampuchea by dispatching troops along the Sino - Vietnam border , and within ten days had reached the provincial capitals . Fighting bogged down due to the maze of tunnels the Chinese encountered ; nevertheless , the Chinese army captured Cao Bang on 2 March and Lạng Sơn on 4 March , reportedly advancing towards Hanoi at a high speed , though not on good terms with its supply lines . However , the following day , the Beijing regime announced that it would not move deeper into Vietnam , apparently after meeting fierce and unexpectedly harsh resistance by the well trained and experienced Vietnamese forces , supplied with American technology left behind from the Vietnam War . Confident that the threat of invasion had been a success , the Chinese retreated , leaving a path of destruction spanning in their wake . With Chinese support lost after Vietnamese recapitulation , Kampuchea was left to fend for itself . Although Chinese total losses were estimated higher than those of the Vietnamese ( 12 – 50 , 000 compared to the Vietnamese death toll of approximately 10 , 000 ) , the diversion of troops from Kampuchea facilitated a strong resurgence in Khmer Rouge insurgent operations , making it unavoidable for the young PRK regime in Kampuchea to implement conscription and require Vietnam to station a large portion of its army along the Sino - Vietnamese border . Internal resistance to the new regime . When the Khmer Rouge regime was removed from power in January 1979 , the Kampuchean people hoped that peace and liberty would return to their country . This was reinforced by the Constitution of the People ’ s Republic of Kampuchea , proclaimed in 1981 , which specifically stated that Kampuchea is an independent , peaceful state where power belongs to the people . However , there was a deep contrast between what was written within the Kampuchean constitution and reality , because the Kampuchean people began to despair at what they viewed as the Vietnamese occupation of their country , rather than a liberation that had freed them from the brutality of Democratic Kampuchea . That perception was reinforced by the presence of Vietnamese advisers who worked at every level of Heng Samrin ' s Kampuchean Government . In 1986 , for example , there was one Vietnamese adviser for every Kampuchean cabinet minister and one adviser for each one of their three deputy ministers . Furthermore , it was reported that final decisions made by a Kampuchean minister had to receive final approval from the Vietnamese adviser , who usually dictated policies . Opposition to Vietnamese was further fermented by the brutal repressive nature of the Vietnamese and their allies . The PRK and Vietnam enslaved 380 , 000 peasants at the cost of 30 , 000 lives . The PRK regime and Vietnamese troops were estimated in total to have murdered ten of thousands of civilians , possibly more than 100 , 000 . One tactic the Vietnamese and KPRAF used to fight the Khmer Rouge was to withhold food from areas controlled by the Khmer Rouge . Thousands of tons of food provided by international relief organizations spoiled on the docks of Kompong Som . Food sent by aid organisations was often instead used to feed Vietnamese troops and their allies . This contributed heavily to the ongoing famine conditions in Cambodia . Vietnamese troops used illegal chemical weapons to quell resistance to the occupation . Following the invasion there were severe famine conditions in the country primarily a legacy of Khmer Rouge rule . Political analyst Rudolph Rummel estimated that over one million died from all causes from 1979 to 1987 with Craig Etcheson estimating 500 , 000 dead by 1981 from famine alone . To resist the Vietnamese occupation of Kampuchea and the regime which they installed , the Khmer Rouge called on the Kampuchean people to unite and fight the Vietnamese . However , due to the brutality which they had experienced under the deposed regime , many Kampucheans believed that any political movement aimed at restoring national freedom must oppose both the Khmer Rouge and the Vietnamese . In response to such preconditions , two non - communist movements were formed to fight the Vietnamese occupation . The first group , a right - wing and pro - Western organisation , was formed in October 1979 by former Prime Minister Son Sann and was called the Khmer People ’ s National Liberation Front ( KPNLF ) . The KPNLF operated from several refugee camps on the Thai - Cambodian border , where it controlled thousands of civilians . At its peak , the armed branch of the KPNLF were estimated to have between 12 , 000 and 15 , 000 fighters , but a third of that number were lost through fighting and desertions during the Vietnamese dry season offensive of 1984 - 85 . Nonetheless , the KPNLF continued to operate in small groups , harassing the Vietnamese and their Kampuchean allies using guerrilla tactics . The other non - communist organisation was the National United Front for an Independent , Peaceful , Neutral , and Cooperative Cambodia , formed by Sihanouk and known by its French acronym FUNCINPEC . The organization was formed after Sihanouk had severed ties with the Khmer Rouge following his representation on its behalf at the UN Security Council . As the leader of FUNCINPEC , Sihanouk called on the UN General Assembly to expel Khmer Rouge representatives for their crimes while in power and to keep Kampuchea ’ s seat at the UN vacant on the basis that neither the Khmer Rouge nor the Vietnamese - installed PRK had the mandate to represent the Kampuchean people . He also criticised ASEAN for its continued recognition of the Khmer Rouge , and specifically Thailand for enabling Chinese arms shipments to travel through its territory to supply the notorious communist group . Despite the strength , effectiveness and popularity of the KPNLF and the FUNCINPEC , both resistance groups were plagued by internal divisions caused by the lack of unity , leadership struggles , corruption and alleged abuses of human rights . In the early days of the Vietnamese occupation , the Kampuchean resistance groups had limited contact with each other due to their differences . Even though the Khmer Rouge enjoyed widespread international support , by 1980 the organization was under pressure to reform itself from the international community . ASEAN , which had backed the Khmer Rouge throughout their diplomatic confrontations with the PRK regime at the UN General Assembly in 1979 , urged the Khmer Rouge leadership to put its blood - stained image behind it in order to join forces with other non - communist movements . However , the idea of forming an alliance with the Khmer Rouge initially caused a certain degree of uneasiness within the leadership circles of the FUNCINPEC and the KPNLF , because both groups were leery about joining with a communist organization well known for its brutality . Nonetheless , early in 1981 , Sihanouk and Son Sann began engaging in talks with Khieu Samphan , President of the deposed Democratic Kampuchea , to discuss the prospect of forming an alliance . In August 1981 , unity talks between the three organizations appeared to have collapsed as a result of conflicting interests . Sihanouk , who feared the resurgence of the Khmer Rouge , proposed that all resistance groups disarm themselves following the withdrawal of Vietnamese troops from Kampuchea . Meanwhile , Son Sann demanded that the KPNLF be the lead organization within the proposed alliance , and the leaders of the Khmer Rouge " most compromised " by the atrocities in Kampuchea be exiled to China . Against these preconditions , Khieu Samphan reminded his rivals that the autonomy of the Khmer Rouge and Democratic Kampuchea should not be undermined . On 22 November 1982 , Singapore , with the backing of ASEAN , proposed that three organizations form a coalition government with equal decision - making powers within the alliance . Singapore ’ s proposal was welcomed by Sihanouk , who believed it was a fair deal for the non - communist movements . Khieu Samphan , on the other hand , rejected that idea , viewing it as an attempt by Sihanouk and Son Sann to isolate the Khmer Rouge . However , Sihanouk knew that Chinese support would not be made available to the FUNCINPEC unless he made some compromises and joined the Khmer Rouge on their terms . So , in February 1982 , Sihanouk met with Khieu Samphan in Beijing to work out their differences . In what he described as " another concession " , Khieu Samphan proposed forming a coalition government without integrating the other resistance groups into institutions associated with Democratic Kampuchea . However , he emphasized that all parties must defend the legal status of Democratic Kampuchea as the legitimate state representing Kampuchea on the world stage . In May 1982 , with the urging of Sihanouk , Son Sann decided to form a coalition government with the Khmer Rouge . On 22 June 1982 , leaders of the three organizations formalized the formation of their coalition government by signing a Thai - sponsored agreement which established the Coalition Government of Democratic Kampuchea ( CGDK ) . Accordingly , the CGDK ’ s Inner Cabinet consisted of Sihanouk as the President of Democratic Kampuchea , Khieu Samphan as the Vice - President in charge of foreign affairs and Son Sann as Prime Minister . Below the Inner Cabinet were six separate committees responsible for national defence , economy and finance , social affairs and public health , military affairs and the media . By 1987 , Democratic Kampuchea still held its membership at the UN General Assembly , even though it lacked four criteria of statehood : people , territory , government and supreme authority within the borders of a country . In spite of those limitations , forces of the three armed factions within the CGDK continued to fight the Vietnamese Army to achieve their objective of “ bring about the implementation of the International Conference on Cambodia and other relevant UN General Assembly resolutions ” . Vietnamese reform and withdrawal . In 1978 , when Vietnamese leaders launched their invasion of Kampuchea to remove the Khmer Rouge regime , they did not expect a negative reaction from the international community . However , the events that followed the invasion showed that Vietnamese leaders had severely miscalculated international sympathies towards their cause . Instead of backing Vietnam , most United Nations member countries denounced the Vietnamese use of force against Kampuchea , and even moved to revive the battered Khmer Rouge organisation that had once governed the country with such brutality . Thus , more than just a military problem , Kampuchea quickly evolved into an economic and diplomatic problem for Vietnam on the international arena . Throughout the decade in which Vietnam occupied neighbouring Kampuchea , the Vietnamese Government , and the PRK regime which it installed , were placed on the periphery of the international community . The international community ’ s political stance towards Kampuchea had a severe impact on the Vietnamese economy , which was already wrecked by decades of continuous conflicts . The United States , which already had sanctions in place against Vietnam , convinced other countries of the United Nations to deprive Vietnam and the People ’ s Republic of Kampuchea of much - needed funds by denying them membership to major international organisations such as the World Bank , the Asian Development Bank and the International Monetary Fund . In 1979 Japan stepped up the pressure by suspending all economic aid to Vietnam , and warned Vietnamese leaders that economic aid would only resume when Vietnam amended its policies towards Kampuchea , the Sino - Soviet rivalry and the problem of the boat people . Sweden , which was considered the staunchest supporter of Vietnam in the West , also considered reducing its commitments to the communist country as virtually every other country cancelled its aid . In addition to external pressure , domestic policies implemented by the Vietnamese Government since 1975 had proven to be largely ineffective in stimulating the country ’ s economic growth . By building on the Soviet model of central economic planning , Vietnam placed most emphasis on the development of heavy industries , while production in agriculture and light manufacturing sectors stagnated . Furthermore , attempts to nationalise the economy of southern Vietnam after reunification only resulted in chaos , as economic output were driven down by dislocation of the general population . In addition to those failed economic policies , Vietnam maintained the fifth largest armed forces in the world , with 1 . 26 million regular soldiers under arms . Consequently , the Vietnamese Government had to spend one - third of its budget on the military and the campaign in Kampuchea , despite receiving $ 1 . 2 billion in military aid annually from the Soviet Union , thus further hampering Vietnam ’ s economic rebuilding efforts . To avoid engaging in a debilitating conflict with various local armed resistance groups within the context of international pressure , Vietnam began withdrawing its military forces from Kampuchea as early as 1982 . But the withdrawal process conducted by Vietnam lacked international verification , so foreign observers simply dismissed Vietnam ’ s movement of troops as mere rotations . In 1984 , in order to disengage from Kampuchea , Vietnam unveiled a five - phase strategy known as the K5 Plan , which was authored by General Le Duc Anh , who had led the Vietnamese campaign in Kampuchea . The first phase required the Vietnamese military to capture the bases of armed groups in western Kampuchea and along the border with Thailand . The following phases included sealing off the border with Thailand , destroying local resistance groups , providing security for the population , and building - up the Kampuchean People ’ s Revolutionary Armed Forces . Foreign observers believed that the Vietnamese Army completed the first phase of the K5 Plan during the dry season offensive of 1984 - 85 , when the base camps of several anti - Vietnamese resistance groups were overrun . Afterwards , the majority of ten Vietnamese divisions were assigned to operations on the frontiers , with the remainder staying in major provinces to protect the local population and to train the Kampuchean armed forces . By 1985 , international isolation and economic hardships had forced Vietnam to rely more and more on the Soviet Union for help . During the Chinese invasion in February 1979 , the Soviet Union provided $ 1 . 4 billion worth of military aid to Vietnam , a figure that peaked at $ 1 . 7 billion in the period between 1981 and 1985 . Then , to help Vietnam implement its third Five Year Plan ( 1981 – 1985 ) , the Soviet Union provided a sum of $ 5 . 4 billion to the Vietnamese Government for its expenditures ; economic aid ultimately reached $ 1 . 8 billion annually . The Soviet Union also provided 90 percent of Vietnam ’ s demand for raw materials and 70 percent of its grain imports . Even though the figures suggest the Soviet Union was a reliable ally , privately Soviet leaders were dissatisfied with Hanoi ' s handling of the stalemate in Kampuchea and resented the burden of their aid program to Vietnam as their own country was undergoing economic reforms . In 1986 , the Soviet Government announced that it would reduce aid to friendly nations ; for Vietnam , those reductions meant the loss of 20 percent of its economic aid and one - third of its military aid . To reengage with the international community , and to deal with the economic challenges brought by the changes in the Soviet Union and Eastern Europe , Vietnamese leaders decided to embark on a series of reforms . At the 6th National Party Congress in December 1986 , newly appointed General Secretary of the VCP Nguyen Van Linh introduced a major reform known as Doi Moi , the Vietnamese term for " renovation " , in order to fix Vietnam ’ s economic problems . However , Vietnamese leaders concluded that Vietnam ’ s dire economic situation came as a result of the international isolation which followed its invasion of Kampuchea in 1978 , and that for Doi Moi to be successful it needed radical changes in defence and foreign policy . Subsequently , in June 1987 , the Vietnamese Politburo adopted a new defence strategy in Resolution No . 2 , calling for the complete withdrawal of Vietnamese soldiers from international duties , a reduction in the size of the army through a discharge of 600 , 000 soldiers and the establishment of a set ratio for military expenditures . Then , on 13 May 1988 , the Vietnamese Politburo adopted Resolution No . 13 on foreign policy , which aimed to achieve diversification and multilateralisation of Vietnam ' s foreign relations . Its main objectives were to end the embargoes imposed by UN members , integrate Vietnam with the regional and international community and ultimately attract foreign investment and development aid . As part of this change , Vietnam ceased to regard the United States as a long - term foe and China as an imminent and dangerous enemy . In addition , official Vietnamese propaganda stopped labelling ASEAN as a " NATO - type " organisation . To implement the new reforms , Vietnam , with support from the Soviet Union , started transferring several years ' worth of military equipment to the Kampuchean People ' s Revolutionary Armed Forces ( KPRAF ) , which numbered more than 70 , 000 soldiers . The Vietnamese Ministry of Defense ’ s International Relations Department then advised its Kampuchean counterparts to only use the available equipment to maintain their current level of operations , and not to engage in major operations which could exhaust those supplies . In 1988 , Vietnam was estimated to have about 100 , 000 troops in Kampuchea , but , sensing that a diplomatic settlement was within reach , the Vietnamese Government began withdrawing forces in earnest . Between April and July 1989 , 24 , 000 Vietnamese soldiers returned home . Then , between 21 and 26 September 1989 , after 15 , 000 soldiers were killed and another 30 , 000 wounded during the 10 - year occupation , Vietnam ’ s commitment to Kampuchea was officially over , when the remaining 26 , 000 Vietnamese soldiers were pulled out . However , armed resistance groups opposed to the Vietnamese - installed PRK regime claimed that Vietnamese troops were still operating on Kampuchean soil long after September 1989 . For example , non - communist groups engaging in land - grab operations in western Kampuchea after the withdrawal reported clashes with elite Vietnamese Special Forces near Tamar Puok along Route 69 . Then , in March 1991 , Vietnamese units were reported to have re - entered Kampot Province to defeat a Khmer Rouge offensive . Despite such claims , on 23 October 1991 , the Vietnamese Government signed the Paris Peace Agreement , which aimed to restore peace in Kampuchea . Aftermath . Paris Peace Agreement . On 14 January 1985 , Hun Sen was appointed Prime Minister of the People ' s Republic of Kampuchea and began peace talks with the factions of the Coalition Government of Democratic Kampuchea . Between 2 – 4 December 1987 , Hun Sen met with Sihanouk at Fère - en - Tardenois in France to discuss the future of Kampuchea . Further talks occurred between 20 – 21 January 1988 , and Hun Sen offered Sihanouk a position within the Kampuchean Government on the condition that he returned to Kampuchea straightaway . However , Sihanouk did not accept the offer , even as preparations were made in Phnom Penh to receive him . Despite that failure , Hun Sen ’ s Kampuchean Government was able to persuade Cheng Heng and In Tam , both ministers in Lon Nol ’ s regime , to return to Kampuchea . In the first major step towards restoring peace in Kampuchea , representatives of the CGDK and the PRK met for the first time at the First Jakarta Informal Meeting on 25 July 1988 . In that meeting , Sihanouk proposed a three - stage plan , which called for a cease - fire , a UN peacekeeping force to supervise the withdrawal of Vietnamese troops and the integration of all Kampuchean armed factions into a single army . Vietnamese Foreign Minister Nguyen Co Thach urged all parties involved to separate Kampuchean problems into internal and external aspects . Therefore , to begin the process of restoring peace , the Vietnamese delegation proposed a two - stage plan that begins with internal discussions among the Kampuchean factions , followed by a roundtable discussion with all involved countries . The Vietnamese proposal won out at the meeting , but no agreements were reached . At the Second Jakarta Meeting , on 19 February 1989 , Australian Foreign Minister Gareth Evans forwarded the Cambodian Peace Plan to bring about a ceasefire , a peacekeeping force and the establishment of a national unity government to maintain Kampuchea ’ s sovereignty until elections were held . To facilitate a peace agreement on the eve of the Vietnamese withdrawal , between 29 – 30 April 1989 , Hun Sen convened a meeting of the National Assembly to adopt a new constitution , and the country was renamed the to reflect the state of ambiguity of the country ’ s sovereignty . Furthermore , Buddhism was re - established as the state religion , and citizens were guaranteed the right to hold private property . In the meantime , however , peace talks between the warring factions continued , with the First Paris Peace Conference on Cambodia held in Paris in 1989 . On 26 February 1990 , following the withdrawal of Vietnamese troops , the Third Jakarta Informal Meeting was held , at which the Supreme National Council was established to safeguard Cambodian sovereignty . Initially , the Supreme National Council was to have 12 members , with three seats allocated to each faction of the CGDK , and three to the pro - Vietnam Kampuchean People ’ s Revolutionary Party . However , Hun Sen objected to the proposed arrangement , calling instead for each faction of the CGDK to be given two seats for a total of six , and the Kampuchean People ’ s Revolutionary Party to have six seats . In 1991 the Supreme National Council began representing Cambodia at the UN General Assembly . Then , in a bold move , Hun Sen renamed the Kampuchean People ' s Revolutionary Party to the Cambodian People ' s Party in an effort to portray his party as a democratic institution and renounce its revolutionary struggle . On 23 October 1991 , the Cambodian factions of the Supreme National Council , along with Vietnam and 15 member nations of the International Peace Conference on Cambodia , signed the Paris Peace Agreement . For the Cambodian people , two decades of continuous warfare and 13 years of civil war seemed to be over , although an atmosphere of uneasiness amongst the leaders of the Cambodian factions remained . In order to include the Khmer Rouge in the agreement , the major powers agreed to avoid using the word " genocide " to describe the actions of the Government of Democratic Kampuchea in the period between 1975 and 1979 . As a result , Hun Sen criticised the Paris Agreement as being far from perfect , as it failed to remind the Cambodian people of the atrocities committed by the Khmer Rouge regime . Nonetheless , the Paris Agreement established the United Nations Transitional Authority in Cambodia ( UNTAC ) , in accordance with the UN Security Council ’ s Resolution 745 , and gave UNTAC a broad mandate to supervise main policies and administration works until a Cambodian government was democratically elected . On 14 November 1991 , Sihanouk returned to Cambodia to participate in the elections , followed by Son Senn , a Khmer Rouge official , who arrived a few days later to set up the organisation ’ s electoral campaign office in Phnom Penh . On 27 November 1991 , Khieu Samphan also returned to Cambodia on a flight from Bangkok ; initially he had expected his arrival to be uneventful , but as soon as Khieu Samphan ' s flight landed at Pochentong Airport , he was met by an angry crowd which shouted insults and abuses at him . As Khieu Samphan was driven into the city , another crowd lined the route towards his office and threw objects at his car . As soon as he arrived at his office , Khieu Samphan entered and immediately telephoned the Chinese Government to save him . Shortly afterwards , an angry mob forced its way into the building , chased Khieu Samphan up the second floor and tried to hang him from a ceiling fan . Eventually , Khieu Samphan was able to escape from the building by a ladder with his face bloodied , and was immediately taken to Pochentong Airport , where he flew out of Cambodia . Thus , with the departure of Khieu Samphan , the Khmer Rouge ’ s participation in the election seemed doubtful . In March 1992 , the start of the UNTAC mission in Cambodia was marked by the arrival of 22 , 000 UN peacekeepers , which included troops from 22 countries , 6 , 000 officials , 3 , 500 police and 1 , 700 civilian employees and electoral volunteers . The mission was led by Yasushi Akashi . In June 1992 , the Khmer Rouge formally established the National Union Party of Kampuchea , and announced that it would not register to participate in the upcoming elections . Furthermore , the Khmer Rouge also refused to disarm its forces in accordance with the Paris agreement . Then , to prevent ethnic Vietnamese from taking part in the elections , the Khmer Rouge started massacring Vietnamese civilian communities , causing hundreds of thousands of Vietnamese to flee Cambodia . Towards the end of 1992 , Khmer Rouge forces advanced into Kampong Thom in order to gain a strategic foothold , before UN peacekeeping forces were fully deployed there . In the months leading up to the elections , several UN military patrols were attacked as they entered Khmer Rouge - held territory . Despite ongoing threats from the Khmer Rouge during the elections , on 28 May 1993 , FUNCINPEC won 45 . 47 percent of the vote , against 38 . 23 percent for the Cambodian People ’ s Party . Though clearly defeated , Hun Sen refused to accept the results of the election , so his Defense Minister , Sin Song , announced the secession of the eastern provinces of Cambodia , which had supported the Cambodian People ’ s Party . Prince Norodom Ranariddh , leader of FUNCINPEC and son of Sihanouk , agreed to form a coalition government with the Cambodian People ’ s Party so the country would not break up . On 21 September 1993 , the Cambodian Constituent Assembly approved a new Constitution and Ranariddh became First Prime Minister , and he appointed Hun Sen as the Second Prime Minister . On 23 September 1993 , the constitutional monarchy was restored with Norodom Sihanouk as the head of state . In July 1994 , the Cambodian Government outlawed the Khmer Rouge for its continuous violations of the Paris Agreement . Most significantly , the Cambodian Government also specifically recognised the genocide and atrocities which occurred under Democratic Kampuchea . By 1998 , the Khmer Rouge was completely dissolved . Vietnam rejoins the world . The military occupation of Kampuchea had profound consequences for Vietnamese foreign policy . Since gaining independence in 1954 , the Vietnamese communist perspective on foreign policy had been dominated by the need to maintain a world order of two camps , communist and non - communist . Indeed , the treaty of friendship that Vietnam signed with the Soviet Union , Laos and the People ’ s Republic of Kampuchea was consistent with that view . However , the ideological motivations of the Vietnamese communist leadership was proven to be limited and heavily flawed , as demonstrated by the 1979 condemnation of Vietnam after the collapse of the Khmer Rouge regime . In the years that followed , the Vietnamese Government was left isolated from the world and its efforts to rebuild the country were handicapped by the lack of aid from the capitalist Western nations . Furthermore , the presence of Vietnamese military forces in Cambodia became an obstacle which prevented the normalisation of diplomatic ties with China , the United States and the member nations of ASEAN . In light of the decline experienced by the Soviet Union and the socialist countries of Eastern Europe , the Vietnamese Government began repairing diplomatic relations with neighbouring countries as part of a greater effort to rejuvenate Vietnam ' s shattered economy . Since its invasion in 1979 , China had placed sustained pressure on the northern borders of Vietnam , with the province of Ha Tuyen regularly shelled by Chinese artillery . In September 1985 , Chinese bombardment of Ha Tuyen reached a peak when 2 , 000 rounds were fired . To reduce the state of hostility along the border region , and ultimately normalise relations with China , the Vietnamese Government dropped all hostile references to China at the 6th National Party Congress in December 1986 , and also adopted the Doi Moi policy . In August 1990 , as the Cambodian Peace Plan , authored by Australian Foreign Minister Gareth Evans , was being endorsed by the UN Security Council , both China and Vietnam moved towards accommodation . Early in September 1990 , Vietnamese Prime Minister Do Muoi , General Secretary Nguyen Van Linh and former Prime Minister Pham Van Dong travelled to Chengdu , China , where they held a secret meeting with Chinese Prime Minister Li Peng and General Secretary of the Chinese Communist Party Jiang Zemin . On 17 September 1990 , General Vo Nguyen Giap also made a trip to China and thanked the Chinese Government for its past assistance . Despite outwards signs of improvement in Vietnam ' s diplomatic relations with China , Vietnamese leaders were reluctant to endorse any peace plan which could weaken their client regime in Phnom Penh . However , as the four Cambodian factions reached an agreement on the power - sharing arrangement outlined at the Third Jakarta Informal Meeting in February 1990 , Vietnam and China rapidly moved to re - establish formal diplomatic relations . In November 1991 , newly elected Vietnamese Prime Minister Vo Van Kiet travelled to Beijing and met his Chinese counterpart , Li Peng , and they issued an 11 - point communiqué re - establishing diplomatic ties between the two countries after 10 years without formal relations . The end of the Cambodian conflict also brought an end the ASEAN - imposed trade and aid embargo which had been in place since 1979 . In January 1990 , Thai Prime Minister Chatichai Choonhavan publicly voiced his support for Vietnam , and the rest of Indochina , to gain admission into ASEAN . In the period between late 1991 and early 1992 , Vietnam restored relations with several member nations of ASEAN . As a result , between 1991 and 1994 , investments from ASEAN countries made up 15 percent of direct foreign investment in Vietnam . Aside from the obvious economic benefits , ASEAN also provided a peaceful environment that guaranteed Vietnam ' s national security against foreign threats in the post - Cold War era , when Soviet aid was no longer available . Thus , on 28 July 1995 , Vietnam officially became the seventh member of ASEAN , after leading ASEAN officials invited Vietnam to join at the ASEAN Ministerial Meeting in Bangkok in 1994 . Then , in August 1995 , the U . S . Liaison Office in Hanoi was upgraded to Embassy status , after U . S . President Bill Clinton announced a formal normalisation of diplomatic relations with Vietnam on 11 July 1995 , thereby ending Vietnam ’ s isolation from the world .