Battle of Dien Bien Phu, French defeat and Beginning of US involvement in Vietnam.

Douglas SBD Dauntless in French service (flickr).

After WWII the French colonial empire collapsed, the French territories in Southeast Asia had been occupied by Japanese forces in 1940, causing the formation of fighting movements against the Japanese, some of which were supported by OSS from the USA. Many of these movements were nationalist and communist like the Vietminh of Ho Chi Minh, but the OSS officers were not interested in this, only that they fight against the Japanese forces. After the Allied victory in WWII, the nationalists led by Ho Chi Minh had declared Vietnam's independence on September 2, 1945, but it was only a symbolic act as the first French troops supported by British and some Japanese troops had begun to occupy parts of the country.  Due to their initial victories on the battlefields, the French began to underestimate the vietminh, which, with Chinese support, became increasingly strong and by 1954 had inflicted severe blows on the French forces.

OSS Officers and Ho Chi Minh, somewhere in the Vietnamese jungle, 1945. (flickr)

The USA was in a difficult situation, as they were sympathetic to the idea of decolonization, of seeking the freedom of nations. But at the same time in Washington they feared the spread of communism in Southeast Asia. The fear of the expansion of communism caused the U.S. to cover much of the French war effort in Indochina with supplies, weapons and money. In 1954 the war was not going well for the French, leadership problems, wrong strategies, and poor results on the battlefield caused many generals to take command in a short time.  General Navarre took over the French command at a difficult time, he wanted an open confrontation with the vietminh, so he planned an operation where he would lure them, to an isolated area, where he thought he could use air and artillery power to defeat them, that strategy would work only if the Vietminh was not able to carry large quantities of supplies and heavy equipment (artillery, above all). He chose an area in the north of the country, the Dien Bien Phu valley (near the Chinese border) surrounded by high hills, and a large plain. There were 15,000 French soldiers and legionnaires facing a force of more than 100,000 viet minh fighters. Faced with this situation the French forces were isolated (destroying the airfield) by the Vietnamese and then defeated. Two American pilots who were on missions over Dien Bien Phu died here. This French defeat meant for the US the beginning of its intervention in Vietnam, first sending supplies, then military advisers and finally combat troops, +500K at its peak. Causing the death of 58K American soldiers, and dividing the country. 

Captured French soldiers, escorted by Vietnamese troops, walk to a prisoner-of-war camp in Dien Bien Phu. (wikipedia)

Sourse

Analogies at War: Korea, Munich, Dien Bien Phu, and the Vietnam Decisions of 1965. Princeton University Press, 

My own investigation.