Well, it didn't. The U.S. entered World War II because the Japanese attacked Pearl Harbor and Clark Field, and because Hitler declared war on the United States.(posted 8771 days ago)At a perhaps deeper level, Roosevelt certainly wanted to see Japan and Germany declare war on the U.S.--and took steps like cutting off oil exports to Japan and having U.S. navy ships tell British ships where German submarines were--because he thought (correctly) that they were threats to America and to the world.
Anyone except the most convinced isolationist would have to agree. Sooner or later the world would have gone nuclear even without the crash effort of the Manhattan Project. Can you imagine a nuclear-armed 1960s in which the Japanese generals whose troops slaughtered more than five million Chinese civilians rule East Asia, and in which Adolf Hitler rules from Gibralter to the Aral Sea, from the North Cape to the Cape of Good Hope?
Brad DeLong