This is a really conplex question that’s super well documented down a thousand rabbitholes. But I’ll attempt to summarizes. Some of this may be insulting to some or surprising to others, but it’s important to think of all this through the context of the cultures of the day.

In very broad terms, we were involved in WW2 long before Japan attacked Pearl Harbor. That became the flashpoint that allowed the politicians to openly propose entering the war to a public that was now super angry. But we were already supporting in a few key ways (arms, lend-lease, etc).

America always has pockets of people who openly support any belief. There were fairly large (by the era) Nazi gatherings and a long history of open racism and anti-culture bias against whoever came to an area after whoever were already there. All of these also had many (and usually larger) counter protests. Again, all in super broad strokes.

But the largest general sentiments were to either stay out of it because it wasn’t-our-problem isolationism and it being “all the way over there” (we were very late to WW1 even), or support for Britain and France out of kindred spirit camaraderie and “why can’t they just leave we’ll enough alone). Plus of course the folks behind the scenes securing and shipping the arms that knew the truth:

The technical progression just from the beginning of WW1 to the end was enough that the same progression from WW2 beginning to end was going to lead to ICBMs or at least long enough ranged missiles, and likely with nukes, the war was going to hit North America. The Axis knew this too (Zimmerman note, sub attacks). And what America had which Japan and Germany didn’t was basically infinite natural resources. So if they could keep us from entering the war, all the best. And we knew if the Axis did win, they’d get those resources from the conquered territories.

The reasons for the Axis to even start the war are as complex and as deep as the particular question you have. But I hope the above helps.

Eugenics was a common belief among the movers and shakers of many regions of the world. This is a very nuanced question and although I think fighting the war to "stop" the Nazis was important it was not a moral decision by goverment. IBM made a fortune off the Holocaust. Henry Ford was an anti semite and eugenisist. In my opinion moat of this stems from the idea that you can perfect humanity and human societies. It always end in tears for large groups if people.

The USSR, for example, was far more conservative, had a pact with Germany, and even seemed quite sympathetic towards the Axis powers in 1939-1941. However, it is hard to imagine that its alliance with Germany would have lasted forever. In the 19th century, European countries did not have many ideological differences, yet they were constantly at war with each other.