I wasn’t around for WWII, but now don’t think you have this quite right. The US fought against Germany and its allies (and Japan, which had directly attacked the US and continued to fight after Germany surrendered). The war was never against Russia.

Sure, maybe the US could have invaded Russia afterwards, but that would have been a disaster, kind of like how Germany’s invasion of Russia was a disaster and a bit like how Russia’s recent invasion of Ukraine has been a real mess for Russia.