Why did the United States enter World War II?

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The United States entered World War II after the Japanese Navy attacked the U.S. naval base at Pearl Harbor on December 7, 1941. This was the culmination of a long process in which Japanese imperial ambitions in the Pacific and German aggressions in Europe threatened to drag a reluctant nation into war. In the 1930s, the United States responded to German and Japanese aggressions by maintaining its own neutrality. But President Franklin Roosevelt was always skeptical that the United States could remain aloof from the affairs of the rest of the world. In 1939, the United States began to provide military assistance to Great Britain, by then at war with Germany, on a "cash and carry" basis. Later, after Hitler overran Europe, Congress approved a "lend-lease" plan that involved massive loans of cash and war materiel to the British, the Soviet Union, and China, all of which were at war with the Axis Powers. But still the United States remained out of World War II. This changed in 1941 when, in response to the Japanese invasion of Indochina, Roosevelt ordered the cessation of oil exports to Japan, and froze Japanese assets. The Japanese then formulated a plan to attack the American naval forces based at Pearl Harbor.