Why did the United States enter World War II?

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The United States entered World War Two after the Japanese navy attacked the US Navy base at Pearl Harbor, Hawaii. The sneak attack, carried out on a Sunday morning, killed more than 2,400 American servicemen and destroyed dozens of American warships, including battleships, that were moored in the harbor. Following the attack, the United States declared war on Japan, and Germany and Italy, Japan's allies in the three-part "Axis Powers" declared war on the United States. With this attack, the United States was officially at war. It is true, however, that the United States had experienced multiple German attacks on shipping in the Atlantic, and that Franklin Roosevelt was aware that an oil embargo on Japan enacted earlier in 1941 had drawn the nation to the brink of war. Moreover, the nation under Roosevelt had shed its rigid isolationist stance to essentially supply Great Britain in its struggle with the Nazis by late 1940. The nation was, in many ways, already on a war footing by 1941. But it was Pearl Harbor that removed all doubt. It was the cause of American entry into World War II.