Was participation by the us crucial to winning the war?

Answer 1

With the exception of possibly Russia (Soviet Union); Britain, France (which didn't exist anyway...it had been occupied by Germany since 1940), China, etc. could NOT have defeated Germany and Japan by themselves.

Answer 2

Yes most certainly, America was the only power strong enough to defeat Japan other than China. The Japanese would have invaded most of pacific is America did not. Air superiority in Pacific and Europe were a major cause of Allied victory by the US