Final answer:
The United States joined WWII due to Japan's attack on Pearl Harbor in 1941, leading to a declaration of war by the U.S. and subsequent involvement in the conflict.
Explanation:
The United States entered World War II primarily due to Japan's attack on Pearl Harbor on December 7, 1941. This event led to the U.S. declaring war on Japan, and subsequently, Germany, due to an alliance between Germany and Japan through the Tripartite Pact. Additionally, long-standing issues between the U.S. and Japan over power in the Pacific played a role in America's entry into the war.
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