Battle of Route Coloniale 4 The Battle of Route Coloniale 4 ( called Chiến dịch Biên giới [ " Operation Border Zone " ] in Vietnam ) was a battle of the First Indochina War . It took place along Route Coloniale 4 ( RC4 , also known as Highway 4 ) , a road which was used to supply the French military base at Cao Bang . French military traffic along the road had previously been subject to an ongoing series of ambushes during 1947 - 1949 . The battle lasted from 30 September to 18 October 1950 and resulted in a French defeat . Several units of the French army , including some battalion s of the Foreign Legion , were devastated by the Viet Minh and essentially ceased to exist as fighting units . RC4 in the First Indochina War . During the French Indochina War ( 1945 – 1954 ) , French forces attempted to re - establish colonial control of Vietnam , while nationalist forces led by Ho Chi Minh fought for independence . Initially , the Vietnamese guerrilla forces , the Viet Minh , were unsuccessful in dealing with the better - trained and - equipped French forces . Their situation improved in 1949 after the Chinese Communist army of Mao Zedong defeated the Nationalist army led by Chiang Kai - Shek . This gave the Viet Minh forces , now almost completely made up of members of the Vietnam Communist Party , a safe haven for organization and training , as well as an initially sympathetic ally to provide them with arms and logistical support . Vo Nguyen Giap , the military leader of the Viet Minh , launched an offensive against the French in early 1950 . From February to April , his operation " Le Hong Phong I " raged through the Red River Valley , largely giving the Viet Minh control of northwestern Tonkin , near the Chinese border . The area became a Viet Minh stronghold , except for the RC4 highway . On 25 May , 2 , 500 Viet Minh troops overwhelmed the French fortress at Dong Khé , which lay at the strategic center of RC4 , thus cutting the supply line between the French positions at Cao Bang and Lang Son . French parachutists retook Dong Khé on the evening of 27 May and a company of Legionnaires took charge of the fort . Meanwhile , the Viet Minh regular army grew in size and experience . By the beginning of September , it comprised roughly 100 , 000 combatants in 70 battalion s , with another 33 battalions of regional forces ( 40 , 000 men ) as well as some 60 , 000 local support personnel . Giap then began harassing French positions along RC4 in northern Vietnam with mines and ambushes . The French responded by dismantling their small posts along the road and concentrating area forces in the fortified positions at Dong Khé and Cao Bang . Giap planned to launch another assault on French positions in operation " Le Hong Phong II " . On 16 September , five Viet Minh infantry and one heavy weapons battalions attacked Dong Khe . It was then garrisoned by some 300 French troops comprising the 5th and 6th companies of the 2nd battalion of the 3rd Regiment of the French Foreign Legion ( 3rd REI ) . On 18 September , the fort was overrun after bitter fighting , and only 12 survivors escaped to the nearby post at That Khé . 140 Legionnaires had been taken prisoner , the remainder being killed or missing in action . That Khe was quickly reinforced by the Foreign Legion ' s 1st Parachute Battalion ( 1st BEP ) , which parachuted in on 17 September . The 1st BEP waited at That Khe while a force of French colonial troops , the Moroccan 1st and 11th Tabors , assembled at Lang Son . Designated " Groupement Bayard " the combined force comprised 3 , 500 men under the command of Colonel Le Page . The task force launched an intelligence raid , capturing prisoners who said a massive Viet Minh offensive was planned . Meanwhile General Marcel Carpentier , the commander in chief of French Indochina , decided to evacuate Cao Bang . The commander of the Cao Bang fort , Colonel Charton , was ordered to destroy his heavy equipment and motor transport and evacuate towards Dong Khé . The plan was that Groupement Bayard would fight its way north from That Khe and retake Dong Khé , holding it long enough to link up with the Cao Bang group . This group comprised 2 , 600 troops and 500 civilians , the latter mostly pro - French Thai partisan s and their families . Battle of RC4 . On 30 September , Groupement Bayard set out from That Khe , led by the 1 BEP . However , Giap had concentrated ten battalions around Dong Khé , reinforced by a complete artillery regiment , together with the remaining forces from Le Hong Phong I . The Viet Minh rebuffed the French forces , which were forced to pull back and wait for air support . Le Page renewed the attack on 2 October , pushing west to bypass Dong Khé as Viet Minh numbers were overwhelming . Meanwhile Colonel Charton ' s group , led by the 3rd Battalion of 3rd REI , left Cao Bang on 1 October ; contrary to orders he took with him his heavy equipment . The group ' s movement down RC4 was slowed by Viet Minh ambushes . After bitter fighting , they finally abandoned their heavy equipment and linked up with Groupement Bayard in the hills around Dong Khé on 5 October . The French forces were driven into the Coc Xa gorge , where they were completely annihilated by 7 October . Martin Windrow notes that : " Some 130 of the Legion parachute battalion out of the 500 that had jumped emerged from this breakthrough fight ; they had only escaped by clambering down lianas shrouding a 75 ft cliff with their wounded tied on their backs " . In an attempt to support the embattled troops the 1er BEP Replacement Company ( 120 men ) under Lieutenant Loth had been merged with 268 men from 3e BCCP ( " Bataillon Colonial de Commandos Parachutistes " , Parachute Colonial Commando Battalion ) under Captain Cazeaux and they were parachuted into That Khe on 8 October , but over the course of the next week destroyed as well . Only 23 survivors of the 1st BEP , led by Captain Jeanpierre , managed to escape to French lines : it became the first French parachute battalion lost in combat , followed by the 3rd BCCP , of which only 14 soldiers returned unscathed . Aftermath . Of the more than 6 , 000 French soldiers and civilians involved in the operation , only 700 reached French lines . Lang Son , the next French base to the south , was abandoned on 17 October and on 18 October the French command evacuated posts south of Lang Son on RC4 . Panic spread in French - controlled Hanoi and there was talk of an evacuation . Ultimately , however , General Jean de Lattre de Tassigny replaced Carpentier as commander in chief of French Indochina in late 1950 and restored French morale . Notes . Windrow , Martin : " The Last Valley " , 2004