Cambodian coup of 1970 The Cambodian coup of 1970 refers to the removal of the Cambodia n Head of State , Prince Norodom Sihanouk , after a vote in the National Assembly on 18 March 1970 . Emergency powers were subsequently invoked by the Prime Minister Lon Nol , who became effective head of state , and led ultimately to the proclamation of the " Khmer Republic " later that year . It is generally seen as a turning point in the Cambodian Civil War . No longer a monarchy , Cambodia was semi - officially called " " État du Cambodge " " ( State of Cambodia ) in the intervening six months after the coup , until the republic was proclaimed . It also marked the point at which Cambodia became substantially involved in the Second Indochina War , as Lon Nol issued an ultimatum to Vietnamese forces to leave Cambodia . Background . Since independence from France in 1954 , Cambodia had been led by Prince Norodom Sihanouk , whose Sangkum political movement had retained power after winning the 1955 parliamentary election . In 1963 , Sihanouk had forced the National Assembly to approve a constitutional amendment that made him head of state with no fixed term of office . He had retained domestic power through a combination of political manipulation , intimidation , patronage , and careful balancing of left - and right - wing elements within his government ; whilst placating the right with nationalist rhetoric , he appropriated much of the language of socialism to marginalize the Cambodian communist movement , whom he called the " Khmers Rouges " ( " Red Khmers " ) . With the Second Indochina War escalating , Sihanouk ' s balancing act between left and right became harder to maintain . Cross - border smuggling of rice also began to have a serious effect on the Cambodian economy . In the Cambodian elections of 1966 , the usual Sangkum policy of having one candidate in each electoral district was abandoned ; there was a huge swing to the right , especially as left - wing deputies had to compete directly with members of the traditional elite , who were able to use their local influence . Although a few communists within the Sangkum – such as Hou Yuon and Khieu Samphan – chose to stand , most leftists were decisively defeated . Lon Nol , a rightist who had been a longstanding associate of Sihanouk , became Prime Minister . By 1969 , Lon Nol and the rightists were growing increasingly frustrated with Sihanouk . Although the basis for this was partly economic , political considerations were also involved . In particular , the nationalist and anti - communist sensibilities of Lon Nol and his associates meant that Sihanouk ' s policy of semi - toleration of Viet Cong and Vietnam People ' s Army activity within Cambodian borders was unacceptable ; Sihanouk , during his swing to the left in 1963 – 66 , had negotiated a secret arrangement with Hanoi whereby in return for the guaranteed purchase of rice at inflated prices , the port of Sihanoukville was opened for weapons shipments to the Viet Cong . As well as the rightist nationalists , the liberal modernising elements within the Sangkum , headed by In Tam , had also become increasingly alienated by Sihanouk ' s autocratic style . There is evidence that during 1969 Lon Nol approached the US military establishment to gauge military support for any action against Sihanouk . Lon Nol ' s appointee as deputy Prime Minister , Prince Sisowath Sirik Matak – a US - friendly nationalist and leader of the Cambodian business community – is thought to have suggested that Sihanouk should be assassinated , though Lon Nol rejected this plan as " criminal insanity " . Sihanouk himself thought that Sirik Matak ( who he characterised as a jealous rival claimant to the Cambodian throne ) backed by the U . S . Central Intelligence Agency ( CIA ) , and in contact with exiled Sihanouk opponent Son Ngoc Thanh , had suggested the coup plan to Lon Nol in 1969 . CIA involvement in the coup plot remains unproven , and Henry Kissinger later claimed that events would take the US government by surprise , but it seems likely that at least some military intelligence agents were partly culpable . Declassified documents indicate that , as late as March 1970 , the Nixon administration was hoping to garner " friendly relations " with Sihanouk . Overthrow of Sihanouk . In March 1970 , while Sihanouk was touring Europe , the Soviet Union and China , large - scale anti - Vietnamese demonstrations erupted in Phnom Penh . Crowds attacked the North Vietnam ese and Viet Cong ( Provisional Revolutionary Government of the Republic of South Vietnam ) embassies . Sihanouk initially gave a certain degree of support to the demonstrators ; he hoped Moscow and Beijing would pressure North Vietnam to reduce its presence in Cambodia . Indeed , it has even been suggested ( by William Shawcross and others ) that Sihanouk and Lon Nol may have planned the first demonstrations to gain political leverage against Hanoi . The riots , however , escalated beyond the government ' s control – although this was likely done with a degree of encouragement from Lon Nol and Sirik Matak – and the embassy was sacked . Inside , a " contingency plan " was allegedly found for the communists to occupy Cambodia . On 12 March , Sirik Matak cancelled Sihanouk ' s trade agreement with North Vietnam ; Lon Nol closed the port of Sihanoukville to the North Vietnamese and issued an impossible ultimatum to them : all PAVN ( People ' s Army of Vietnam aka . North Vietnamese Army ) and NLF ( National Liberation Front aka . Viet Cong ) forces were to withdraw from Cambodian soil within 72 hours ( on 15 March ) or face military action . When , by the morning of 16 March , it was clear that this demand had not been met , some 30 , 000 youths gathered outside the National Assembly in Phnom Penh to protest against the Vietnamese presence . From this point , events were to move with increasing rapidity . On the same day , the Cambodian Secretary of State for Defence , Colonel Oum Mannorine ( Sihanouk ' s brother - in - law ) , was scheduled to be questioned by the national legislature on allegations of corruption ; the proceedings were adjourned to hear the demonstrators ' resolutions . According to Sihanouk , Mannorine had received information that Lon Nol and Sirik Matak were about to precipitate a coup ; a group of Mannorine ' s men , under the command of Phnom Penh ' s Chief of Police Major Buor Horl , attempted to arrest the plotters , but it was by then too late . Mannorine , and other key security personnel loyal to Sihanouk , were placed under arrest . After the Assembly adjourned for the day , Sihanouk ' s mother Queen Kossamak , at Sihanouk ' s request , summoned Lon Nol and Sirik Matak to the Royal Palace and asked them to end the demonstrations . It appears to have been sometime during 16 or 17 March that Sirik Matak finally swayed Lon Nol to remove Sihanouk from the government . Lon Nol , who until that point may have been merely hoping that Sihanouk would end his relations with North Vietnam , showed some reluctance to take action against the Head of State : to convince him , Sirik Matak allegedly played him a tape - recorded press conference from Paris , in which Sihanouk threatened to execute them both on his return to Phnom Penh . However , the Prime Minister remained uncertain , with the result that Sirik Matak , accompanied by three army officers , finally compelled a weeping Lon Nol to sign the necessary documents at gunpoint . The next day – 18 March – the army took up positions around the capital , and a debate was held within the National Assembly under In Tam ' s direction . One member of the Assembly ( Kim Phon , later to be killed by pro - Sihanouk demonstrators in Kompong Cham ) walked out of the proceedings in protest , though was not harmed at the time . The rest of the assembly voted unanimously to invoke Article 122 of the Cambodian constitution , which withdrew confidence in Sihanouk . Lon Nol took over the powers of the Head of State on an emergency basis , while the position itself was taken by the President of the General Assembly , Cheng Heng . In Tam was confirmed as President of the Sangkum . The coup had , therefore , followed essentially constitutional forms rather than being a blatant military takeover . These events marked the foundation of the Khmer Republic . Demonstrations against the coup . On 23 March , Sihanouk ( via Beijing Radio ) called for a general uprising against Lon Nol . Large - scale popular demonstrations calling for Sihanouk ' s return began in Kompong Cham , Takéo Province , and Kampot Province . The demonstrations in Kompong Cham became particularly violent , with two National Assembly deputies , Sos Saoun and Kim Phon , being killed by demonstrators on 26 March after driving to the town to negotiate . Lon Nol ' s brother , police official Lon Nil , was set upon in the nearby town of Tonle Bet by plantation workers and was also killed . The demonstrations were suppressed with extreme brutality by the Cambodian army ; there were several hundred deaths and thousands of arrests . Some witnesses spoke of tanks being used against crowds of unarmed civilians . Aftermath . Following the coup , North Vietnam forces invaded Cambodia in 1970 at the request of Khmer Rouge leader Nuon Chea . These events would earmark the start of the Cambodian Civil War pitting Lon Nol ' s regime backed by US air power against the Khmer Rouge and North Vietnam .