While this is the wrong place to discuss who won the war, quite a few put their 2 cents in...
Even if we never fired a shot, without US production capabilities, the ships that hauled the equipment, and the Navy that enabled their delivery, neither Russia nor Britain would have made it. Also, we were Japan's primary source of materials prior to the war. Denying Japan the resources she needed to take over Asia is why she declared war on us in the first place.
The U.S. was the only country with both the resources and the resolve to produce enough ships (both war and merchant) fast enough and long enough to control the ocean and thereby control the world to this very day. Germany, without a real Navy was doomed to control only mainland Europe (they couldn't keep Africa supplied) and Japan without its own steel could only fight a short war.
By no means did the U.S. win the war by itself. Almost the entire world struggled against the Axis while the U.S. lazily got its war machine in full gear. In both theaters, costly fighting retreats bought the U.S. the time it needed to build up the production capability needed to turn the tide against the Axis.
Stalingrad was the pivotal point in Europe, much more important than D-Day. But if the U.S. had allowed Britain to fall, the full might of Germany might have overcome the Russians.
In my opinion, acting alone the U.S. would have been able to hold out for such a long time that it could have negotiated a peace with the axis, or even singlehandedly fought and defeated both of them. But speculation is meaningless. All that matters is the end result. Anyone on mainland Europe and Asia paid dearly in lost lives. Only people living in places isolated by oceans were spared death, imprisonment, and/or subjugation.
As for the inefficiency of Russians in combat, our data is largely based on German records which were previously assumed to be very accurate, but have recently been determined to be nearly as unreliable as our own inflated combat reports.
For a lesson in U.S. propaganda, try researching our kill ratio in Korea against MiGs, I can assure you it wasn't even close to the U.S. claim of 10 to 1 (which was only true during a limited period of the war).
