Final answer:
The United States declared war on Japan as a direct response to the Attack on Pearl Harbor carried out by Japan on December 7, 1941. This significant event shifted the United States from a position of neutrality to active engagement in World War II.
Step-by-step explanation:
The event that led to the United States declaring war on Japan during the second World War was the Attack on Pearl Harbor. This surprise military strike by the Imperial Japanese Navy Air Service upon the United States against the naval base at Pearl Harbor in Hawaii was a pivotal incident that stirred the nation. On the very next day, December 8, 1941, President Roosevelt addressed the nation and urged Congress to declare war on Japan. This attack abruptly ended the stance of neutrality the U.S. had held and led it into World War II, a global conflict that spanned over several years and multiple continents.
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