hot military history take 1;
The USA's biggest contribution to WWII was logistics. During the 3 years we weren't involved, we had a manufacturing base which wasn't being relocated or bombed, rubber was our biggest shortage, potential soldiers didn't immediately go into the meat grinder, our technology was more refined (compare Commonwealth lend-lease Grants to Americans starting the war in Shermans).
This is HUGE vital stuff but of course it doesn't sound as awesome as America Saves Everyone.