The United States did not send as many troops and did not join the war until very late.
Explanation:
Germany starting WW2 meant to take out France, making the UK weak. Originally, Hitler's plan was to team with the Soviets, defeat France, knocking out the UK and turn on the Soviets. United States allies sent over 2M+ troops. The allies were also fighting from the very beginning.
The United States tried to limit North Vietnam's power as they were afraid of a domino effect occurring and the spread of communism into other neighboring nations occurring