Necessary: The joining of the US in WW2.
Without the US joining the war, most of the nations that were part of the allied powers would have fallen, leaving the US to fight two fronts by itself at all times. This meant that more resources would be drained, more loss of life, and the war would have continued on longer.
Unnecessary: US fighting the Vietnam War.
Sure, the US defended all nations that were against Communism, but it does not mean they should support totalitarian states. Them choosing to join the war was wrong, because the people did not support them (1), joining a war just because of a ship attack isn't the best idea (2), They were not even prepared for war (3). You may argue that they weren't prepared for WW2, however, they were, with people joining foreign armies (British), making equipments already (Lend-Lease Program), and already had many people ready to join the war (Roosevelt's New Deal) in which many young Americans were already work-ready.
hope this helps
