Which event directly prompted the United States' entry into World War II?

The attack on Pearl Harbor is the event that directly prompted the United States' entry into World War II because it was a sudden and aggressive act of war committed by Japan on December 7, 1941. This surprise military strike targeted the U.S. naval base at Pearl Harbor in Hawaii and led to significant loss of life, as well as the destruction of numerous naval vessels and aircraft. The attack galvanized public opinion in the United States, which had been largely isolationist and reluctant to enter the conflict. In the wake of this attack, President Franklin D. Roosevelt and Congress swiftly declared war on Japan, marking the United States' official engagement in World War II. The attack not only united the American people in support of the war effort but also shifted the course of U.S. foreign policy from isolationism to active involvement in global affairs.