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The U.S. contributions to World War II fell under three broad categories:
In the 1945 book, the main U.S. contribution to the war was the defeat of Japan (with China becoming a "third world" power aligned neither with the United States nor Nazi Germany). That was basically a minimum condition for American survival.
The hypothesis from my unpublished World War II book "Axis Overstretch" is that the Axis win if they ever obtain control of 50% or more of the world's industrial capacity. According to Paul Kennedy's "Imperial Overstretch," North and South America had just under 45% of the world's industrial capacity in the 1940s. Add Australia, Japan and the latter's holdings in the ASEAN nations of Southeast Asia, and you get just over 50% for the U.S.-led bloc.
The 1945 book presupposed the failure of American Lend Lease aid and the resulting inability of America to protect Britain and the Soviet Union. In the "real" war, American Lend Lease aid approximated the total Axis output, thereby giving America's allies a meaningful preponderance of material against the Axis, which was enough for them to survive.
The introduction of American military forces in western Europe was the "icing on the cake," that sealed the fate of Nazi Germany. Even without this, the more pressing need was to ensure the survival of Britain and the Soviet Union, and the defeat of Japan. Britain and the Soviet Union had a bare preponderance of strength against Nazi Germany alone (the danger of U.S. non-intervention was that Japan would tip the balance in Germany's favor). Throw in U.S. power and the "bare preponderance" of Allied power in Europe becomes overwhelming.