The US did not enter World War II when Nazi Germany overran eastern Europe and reached the outskirts of Moscow and Leningrad.
It did not move to stop Japanís conquest of Manchuria and the great cities of China.
But the fall of France in May 1940, the prospect of German victory in Russia, and Japanís aggressive extension of its Asian empireótaken togetheróthreatened to isolate the United States in a hostile, militarized world.
Recognizing the danger, President Roosevelt encouraged public support for rearmament and Congress responded. The US also cut off Japanís fuel supplies and demanded its withdrawal from China.
Rather than give up its empire, Japan preemptively attacked Pearl Harbor. Hitler (who had encouraged the Japanese attack) followed suit by declaring war on the United States.
The US responded by a full mobilization and a commitment to unconditional victory. By 1944, when victory was in sight, the US initiated ambitious plans for post-war reconstruction, a generous peace designed to integrate both allies and former enemies into a US-led global order.