Final answer:
The bombing of Pearl Harbor on December 7, 1941, led to the United States joining WWII.
Step-by-step explanation:
The event that happened on December 7, 1941, which led to the United States taking part in WWII, was the bombing of Pearl Harbor.
On that day, the Japanese carried out a surprise military attack on the U.S. naval base at Pearl Harbor in Hawaii. This attack resulted in the deaths of over 2,400 Americans and caused extensive damage to the U.S. Pacific Fleet.
As a response to this attack, the United States declared war on Japan, officially entering World War II.
Learn more about The bombing of Pearl Harbor