To be fair, none of the major powers fought in WWII because of racism or ideology really. All of that was just a pretense, and a great deal of it was post-war justification. Most of the nations involved fought for resources, territory, or just influence over a particular region of the globe. Some of the different Slavic and Jewish groups in Eastern Europe definitely fought against racism to the extent that they were fighting for survival because the Nazis were going to kill most or all of them if they lost. But I wouldn't characterize the groups that were fighting against extinction as major powers.

The Americans entered the war in Europe because their closest allies in Europe had either capitulated or were fighting a losing war, they saw an opportunity to gain more influence and international ties in Europe, an unstable, aggressive and unreasonable dictator controlling most of Europe was not in their best interest (or anyone else's, maybe not even Germany in the long run) and Hitler supported an unprovoked Japanese attack on American soil. They literally waited until the last possible moment to join the war, and to not declare war on the Germans at that point would have made them look impotent, ignorant and negligent on a global scale.