The US became involved in the French Colony of Vietnam following WW2 because they wanted to stop the spread of communism. France was a democratic country, and the guerillas fighting them were mostly communists, backed by the USSR. The US first became involved in helping supply the French, and later on was given South Vietnam to look after (setting up a democratic government) Then the North Vietnamese decided to reunite the country under Communist rule, and the US fought an unpopular war (leading to their withdrawal and then the loss of South Vietnam) and there you go, Vietnam today.
hope this helps