During 1954 to 1965, America became more involved for several important reasons, the most important one is that America feared Vietnam would expand communism all over Asia. Throughout these years the USA could have retreat and stopped giving aid to South Vietnam, but didn't, as the Presidents wanted to maintain there stature of strength and not let Russia think they were weaker. .
From 1946 ahead the Vietminh organisation set-up and was led by, communist Ho Chi Minh. He fought seriously with the French. They fought with far more primitive weapons than the French who used modern powerful weapons supplied by the US. But since Vietminh were poor on the subject "technology" they made up with ferocity, anger, patriotism and tactics. They fought using Guerrilla techniques and gave them major advantage to the Vietminh. .
The US was not actually into the war they were just supplying the French. The French primarily fought to preserve their empire disagreed in the reason why America got involved. The USA were fighting against communism and not as war between colonialists and Nationalists. The USA were fighting for the wrong reasons. .
The year 1954 was a major turning point, which brought about the immediate involvement of USA. .
The battle of Dien Bien Phu, in which the French were defeated by the Vietminh. This caused the French to leave Indo-China. The Geneva peace agreement led to the temporary division of Vietnam. This was temporary, the elections were to be held in 1956 in which the Vietnamese could vote for one leader for the tow divisions. .
The Vietminh leader Minh controlled North Vietnam and Bao Dai Controlled South Vietnam; he was disliked and was exiled by Diem. .
Diem was a Nationalist and was supported by the Americans. Diem had no intention of holding the elections and said that the communists would not allow free elections in the North. This broke the Geneva peace agreement and the USA supported his actions.