First Indochina War - Wikipedia

On September 15-18, 1950, Viet Minh undertook an offensive against French bases in Vietnam. From the article:

"By January 1950, Ho's government gained recognition from China and the Soviet Union. In the same year, the government of Bao Dai gained recognition by the United States and the United Kingdom.

In February, Giap seized the vulnerable 150-strong French garrison at Lai Khê in Tonkin just south of the border with China.

"In June, the Korean War broke out between communist North Korea (DPRK) supported by China and the Soviet Union, and South Korea (ROK) supported by the United States and its allies in the UN. The Cold War was turning 'hot' in East Asia, and the American government feared communist domination of the entire region would have deep implications for American interests. The US became strongly opposed to the government of Ho Chi Minh, in part, because it was supported and supplied by China.

Major general Thái attacked Đông Khê on September 15.[62] Đông Khê fell on September 18.

Cao Bằng garrison was then evacuated south, together with the relief force coming from That Khe, were attacked all the way by ambushing Viet Minh forces, which result in a stunning French defeat in the Battle of Route Coloniale 4. The French air-dropped a paratroop battalion south of Cao Bằng to act as diversion only to see it quickly surrounded and destroyed. After that, Lạng Sơn, is evacuated in panic while it wasn't menaced.

By the time the remains of the garrisons reached the safety of the Red River Delta, 4,800 French troops had been killed, captured or missing in action and 2,000 wounded out of a total garrison force of over 10,000. Also lost were 13 artillery pieces, 125 mortars, 450 trucks, 940 machine guns, 1,200 submachine guns and 8,000 rifles destroyed or captured during the fighting. China and the Soviet Union recognized Ho Chi Minh as the legitimate ruler of Vietnam and sent him more and more supplies and material aid. The year 1950 also marked the first time that napalm was ever used in Vietnam (this type of weapon was supplied by the U.S. for the use of the French Aéronavale at the time).

The military situation improved for France when its new commander, General Jean Marie de Lattre de Tassigny, built a fortified line from Hanoi to the Gulf of Tonkin, across the Red River Delta, to hold the Viet Minh in place and use his troops to smash them against this barricade, which became known as the De Lattre Line. This led to a period of success for the French."