What were the causes of American involvement in World War II?

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mrkirschner eNotes educator| Certified Educator

Americans were wary of becoming involved in another international conflict.  Most felt that they were unfairly thrust into World War I for the wrong reasons and hoped to avoid the death and destruction of another war.  For this reason, the 1920's and 1930's was a period of isolationism in the United States.  Japan in its quest to become a modern industrial power, however, was beginning to threaten American interests.  The 1931 invasion of Manchuria deeply threatened the relationship between Japan and the United States.  The United States became concerned because it had a strong relationship with China and felt threatened by Japan's recent aggression.  The Japanese invaded French Indochina in 1940, mostly to stop the flood of American weapons being peddled from the French territory to the Chinese. This action led to the United States placing an oil embargo on Japan.  The United States also froze Japanese assets.  The Japanese viewed both of these actions as acts of aggression because it seriously threatened their military capacity.  It was estimated that Japan only had two years worth of oil reserves.  Japan, feeling cornered by the oil embargo and seeking a Pacific empire, was compelled to attack the United States at Pearl Harbor on December 7, 1941.  This action led to a declaration of war by Franklin D. Roosevelt.