re: Would the Allies have won WWII without America getting involved?

Define "involved".

Britain probably would have been bankrupt, out of fuel, and starving had we stayed completely neutral from 1939 to 1941. The U Boats nearly had them on their as knees as it was by 1940.

Britain probably would have sued for peace (and Germany would have granted it with very favorable terms to the Reich). They then turn their full attention to the Soviets (who, in this theoretical, would not have Lend/Lease benefits), and who knows what happens at that point.

The Japanese are a whole other issue.