The Battle of Cao Bang was a two-year long battle fought between the Viet Minh forces and the French Far East Expeditionary Corps during the French Indochina War. It took place in northern Indochina, from October 1947 to September 1949. The Battle of Cao Bang began with a series of ambushes sprung by Viet Minh troops on French convoys along the Vietnam–China border from the Gulf of Tonkin on a 147-mile-long route to a French garrison at Cao Bang, known as Route Colonial 4, or RC4.
In order to reopen the Route Colonial 4, the French forces carried out several military operations of increasing strength, including a costly mission by the Foreign Legion in February 1948. On July 25, 1948, the Cao Bang French encampment was also attacked and held out for three days. Two companies fought fiercely against two battalions of Viet Minh. During 1948, 28 ambushes took place along the RC4.
In February 1949, five Viet Minh battalions armed with mortars and machine guns captured a French post at Lao Cai, resuming ambushes through the monsoon season. On September 3, 1949, 100 vehicles left That Khe in a reinforced convoy on a 16 miles (26 km) drive through infantry screens. The French, reduced to one-soldier per vehicle due to troop numbers, were ambushed by automatic fire. The first twenty trucks were halted, as were the final ten, and the middle of the convoy was cut down my shellfire. The following day, French troops reoccupied the surrounded hilltops, however only four wounded French soldiers were found alive.
The military engagements at Cao Bang led to a change in French convoy practices for the remainder of the war. Vehicles now traveled from post to post in 10-12 vehicle convoys, through security screens of French troops and with aircraft observation. Through 1950, supply convoys to Cao Bang were discontinued in favour of air supply.
