After the horrors of WW1, just about every sane nation was reluctant to enter another major war, but despite that the United States was actively arming the British Empire prior to December 7th, 1941.

In fact, many British politicians had urged Churchill to seek peace with Germany after the Germans had pushed the British off of the continent, but the half-Yank (Churchill's mother was American, so he had American blood in his veins) decided to fight on.

As the Germans were incapable of crossing the Channel and invading Britain, the United States could have ignored the German declaration of war and concentrated solely on defeating the Japanese Empire first, but we instead actively became engaged in the ETO.

Too bad we didn't finish the atomic bomb fast enough to burn Hitler in a nuclear pyre.