French defeated at Dien Bien Phu

On May 7, 1954, the French surrender to the Vietminh after a 55-day siege at Dien Bien Phu. From the article:

"French defeated at Dien Bien Phu
In northwest Vietnam, Ho Chi Minh’s Viet Minh forces decisively defeat the French at Dien Bien Phu, a French stronghold besieged by the Vietnamese communists for 57 days. The Viet Minh victory at Dien Bien Phu signaled the end of French colonial influence in Indochina and cleared the way for the division of Vietnam along the 17th parallel at the conference of Geneva.

On September 2, 1945, hours after the Japanese signed their unconditional surrender in World War II, communist leader Ho Chi Minh proclaimed the independent Democratic Republic of Vietnam, hoping to prevent the French from reclaiming their former colonial possession. In 1946, he hesitantly accepted a French proposal that allowed Vietnam to exist as an autonomous state within the French Union, but fighting broke out when the French tried to reestablish colonial rule. Beginning in 1949, the Viet Minh fought an increasingly effective guerrilla war against France with military and economic assistance from newly Communist China. France received military aid from the United States.

In November 1953, the French, weary of jungle warfare, occupied Dien Bien Phu, a small mountain outpost on the Vietnamese border near Laos. Although the Vietnamese rapidly cut off all roads to the fort, the French were confident that they could be supplied by air. The fort was also out in the open, and the French believed that their superior artillery would keep the position safe. In 1954, the Viet Minh army, under General Vo Nguyen Giap, moved against Dien Bien Phu and in March encircled it with 40,000 Communist troops and heavy artillery.

The first Viet Minh assault against the 13,000 entrenched French troops came on March 12, and despite massive air support, the French held only two square miles by late April. On May 7, after 57 days of siege, the French positions collapsed. Although the defeat brought an end to French colonial efforts in Indochina, the United States soon stepped up to fill the vacuum, increasing military aid to South Vietnam and sending the first U.S. military advisers to the country in 1959."