Americans did not support WWII 100% at the time. Only in hindsight do people tend to agree that WWII was necessary.
Many of my uncles and cousins fought (one died) in WWII. Yet, many people, including their own parents, opposed sending Americans into war at that time. Even though they themselves were European immigrants with relatives still living across the Atlantic, they viewed themselves as Americans and the war as someone else’s problem.
So, many people opposed WWII back then. The difference was: Back then, most had enough common sense to realize that openly protesting it would embolden the enemy against whom their American sons were drafted to fight.
Of course, the government’s stance was much harsher then, too. Imagine, we weren’t attacked on U.S. soil, yet suspects were rounded up and expatriated or imprisoned. Contrast that with today: After thousands of civilians were attacked on our own soil in a major city, the government tiptoes around suspects and is harshly criticized for the slightest hint of any violation of individual rights.
Communist Party USA was very up front about opposing going to war against Nazi Germany. That is until Hitler betrayed their beloved Joseph Stalin.