The Americans supported the return of the French to SE Asia shortly after the end of WWII, thus double-crossing Ho Chi Minh, who the Americans supported against the Japanese during WWII. The Americans were very concerned that the communists would gain support in France, and were willing to do almost anything to keep the French population friendly........including supporting France in this idiotic case of military adventurism.
The French were given weapons and air support. After their decisive defeat at Dien Bien Phu in 1954, the Americans supported a UN presence and a move towards nation-wide elections to be held in 1958.
It became increasingly obvious Ho Chi Minh would win those elections, and he had earned (in the eyes of the USA) the fatal label "communist", and so the elections never happened, and the temporary divide between North and South became permanent. US advisors trained the ARVN (Army of the Republic of Viet Nam). Kennedy increased aid, and sent "advisors" into combat situations. Johnson sent in the first regular troops in 1965. By Tet in 1968 there were 569,000 US troops in Viet Nam.
