Final answer:
The primary effect of the Japanese attack on Pearl Harbor was that the United States officially entered the war on the side of the Allies.
Step-by-step explanation:
The primary effect of the Japanese attack on Pearl Harbor was that the United States officially entered the war on the side of the Allies. Before the attack, the United States had remained neutral, but the attack on Pearl Harbor on December 7, 1941, led to the U.S. declaring war on Japan the following day. This event marked the entry of the U.S. into World War II.
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