Answer:
Necessary: The joining of the US in WW2.
The majority of the allied countries would have collapsed in the event that the US had not entered the conflict, leaving the US to constantly fight on two fronts by itself. As a result, more resources would have been depleted, more lives would have been lost, and the war would have lasted longer.
Unnecessary: US fighting the Vietnam War.
Yes, the US stood up for all countries who opposed Communism, but that does not mean they should back totalitarian governments. They made the wrong decision to enter the conflict because: (1) The populace did not support them; (2) It is not a good idea to enter a conflict solely as a result of a ship attack; and (3) They were not even equipped for conflict (3). However, they were, with people enlisting in foreign armies (British), producing equipment (Lend-Lease Program), and having many people ready to serve in the war (Roosevelt's New Deal), which had many young Americans who were already employable.
Step-by-step explanation: