The U.S. didn't get militarily involved until Hitler attacked one of our ships. He drew us into the war deliberately. We were non-commital until then, sadly. (If he has never attacked us and invaded Russia, he likely would have owned most of Europe. Attacking 2 Superpowers was a BAD idea.)
The world (U.N.) has learned nothing from this, either. The U.N. refused military action in Serbia when there were concentration camps and ethnic cleansing going on in the 90's, it took NATO to end that by themselves. And look how we stand by when similar horrors occur in Rwanda and Darfur. We've learned nothing from the Holocaust. Nations still stand by while people suffer and die. Oh there may be some token gestures, but that's about it.
Off my sopabox now, lol.