Lots of "Americans" thought what the Germans did was OK; there were different attitudes back then.
We didn't fight the Japanese because they killed millions in China; it was none of our business.
We fought WWII because we had no choice.
Japan attacked us and Hitler then declared war upon us.
There were no options left.
It is nice that we think so kindly and proudly of our predecessors and ancestors, and give them credit for nobly saving the world. They did of course. But only after the job was forced upon them.
Before Dec. 7, 1941 there were active Nazi party agitators in the US; supported by large segments of the German-American community.
There were isolationists. There may well have been pro-Japanese factions--I do not know.
There was plenty with anti-British sentiment and people who couldn't care less if Germany continued to bomb the heck out of the UK.
We probably forced Japan's hand with economic embargoes on steel and oil exports, but I think that was excuse, and not cause.
The war was going to happen because the dictators of Germany, Japan, Italy, like BGs everywhere, knew no bounds to their evil. They brought their evil to our territory, and forced us to act.
Like all BGs, they were dumber than the utter filth they (and their adherents) were. Had they been a little smarter, they'd have figured out a way to keep us neutral, while attaining their goals.