The Battle of Cao Bằng was an ongoing campaign in northern Indochina during the First Indochina War, between the French Far East Expeditionary Corps and the Việt Minh, which began in October 1947 and culminated on September 3, 1949. Since the start of the conflict, Việt Minh troops had ambushed French convoys along the Vietnam–China border from the Gulf of Tonkin on a 147-mile route to a French garrison at Cao Bằng, known as Route Colonial 4, or RC4. Repeated ambushes led to repeated French operations of increasing strength to reopen the road, including a costly mission by the Foreign Legion in February 1948. On July 25, 1948, the Cao Bằng encampment was itself attacked and held out for three days with two companies defending against two battalions of Việt Minh. A further 28 ambushes took