World War II (1939-45) began in September 1939. The United States maintained neutrality—a policy of not taking either side in a dispute. Despite that official policy, government actions on the part of the United States were often supportive of the Allied nations— Great Britain, France, and the Soviet Union —in their struggle against Nazi Germany. Meanwhile, the United States had strained relations with Japan since 1931, when that nation invaded and occupied the Manchuria region of China. Tensions between the United States and Japan grew worse after World War II began and the Japanese army occupied several other countries. In 1941, the United States announced a trade embargo against Japan.

The nations continued to make occasional attempts at negotiation, but that ended on December 7, 1941, when the Japanese military launched a surprise attack on the American...