What was the main event that triggered the U.S. entry into World War II?

The main event that triggered the U.S. entry into World War II was the attack on Pearl Harbor on December 7, 1941. This surprise military strike by the Japanese Navy against the naval base in Hawaii resulted in significant destruction of the U.S. Pacific Fleet, including battleships and aircraft. The attack led to widespread outrage among the American public and prompted President Franklin D. Roosevelt to ask Congress for a declaration of war against Japan the following day, which they granted almost unanimously.

This event marked a crucial turning point in U.S. history, as it shifted public opinion overwhelmingly in favor of entering the conflict after years of isolationist sentiment. The attack fundamentally changed the nature of American foreign policy and military involvement in global affairs, establishing the United States as a major participant in World War II.

While the sinking of the Lusitania and other events contributed to tensions prior to the war, they did not result in an immediate U.S. declaration of war. Moreover, while the declaration of war by Germany against the United States followed shortly after Pearl Harbor, it was the attack itself that directly prompted U.S. involvement in the war. The bombing of London, although significant in the context of the European theater, did not have the same direct impact