The introduction of the tank was the war winner in World War I, not the U.S. The American troops had to be trained by the French and British before even being allowed into the lines. The Germans were on their last legs after the British and French realised how to use tanks effectively at Cambrai. The German forces had no effective anti-tank defence and it wasn't long until the tanks were soon back on the field at Amiens, which the New Zealanders led. The tank won World War 1, end of story.
Pre-World War II the French were going backward in military doctrine. They were out-classed tactically by Germany but not technically. The French forces had some of the best tanks in the world, superior to their German counter-parts. And Germany invaded France on May 10th, France surrendered on June 22nd - that's six weeks not four.
Don't forget the Soviet Unions massive contribution to the war effort. It wasn't just the U.S and Britain.
It was the rebellion in Yugoslavia and the landing of British forces in Greece that forced Germany to turn south and delaying Operation Barbarossa. The Italians could have stopped the British landing but they couldn't have stopped Yugoslavia rebelling. The Italians were just a bunch of failures and get off with that war lightly. I think everyone forgets they were fascist and invaded British terrority, as well as bombing Britain.