Why did America declare war on Japan after Pearl Harbor?

The simple answer is to say that the United States declared war because of the attack on Pearl Harbor. This was a premeditated, aggressive action against the US naval forces at Pearl Harbor, and it was very destructive, killing over 2,400 American servicemen. It was unthinkable that the United States would not declare war on Japan after what was, after all, an act of war. The attack at Pearl Harbor was the culmination of years of escalating tensions between Japan and the United States, which very much saw its interests threatened by Japanese expansion. A few months before Pearl Harbor, the US ceased exports of essential war materials, including oil, to Japan, an act which Japan itself saw as aggressive in nature. Indeed, Pearl Harbor was part of a massive offensive against American interests in the Pacific, one designed to cripple the American presence in the Pacific in one fell swoop. Just hours after the attack in Hawaii, Japanese forces moved against the Philippines, at the time an American territory. So, in short, the United States declared war against Japan because it was clear to everyone after the events of December 7, 1941, that Japan was already at war with the United States.
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