All I want to say about those who think that the British Commonwealth/Empire and the Soviets could win the war by themselves without America is that you can't ignore the elephant in the room; ie US Lend Lease.
On paper the British Commonwealth looks like it had the manpower and resources to take on the Axis with the Soviets, but did it happen? The importance of US Lend Lease to the USSR is well known but a lot less is and has been said about US Lend Lease to Britain. They got over $31 billion worth of material, three times more than the Soviets. US War Department figures are all here.......
http://www.ibiblio.org/hyperwar/USA/...hip/index.html
And don't forget that the US industry supplied Britain, the USSR and other countries with this material while it was building the largest airforce and navy in history, and an army/marine force of 100 well equipped divisions. Lend Lease was an IOU ie pay it back after the war is won when you can manage it. If America stayed neutral then the British, Soviets and everyone else will be paying top dollar for all of this and America will have no obligation whatsoever of selling its best weaponry and technology to anyone.