What event led to the U.S. entering World War II?

The attack on Pearl Harbor is recognized as the critical event that led to the United States entering World War II. On December 7, 1941, the Japanese military launched a surprise attack on the U.S. naval base located at Pearl Harbor in Hawaii. This assault resulted in significant loss of life and destruction of naval equipment and aircraft. The attack galvanized American public opinion and united the nation in support of declaring war against Japan.

Following the attack, President Franklin D. Roosevelt addressed Congress, famously referring to December 7 as "a date which will live in infamy," and called for a declaration of war against Japan. The subsequent declaration led to the U.S. joining the Allies in the fight against the Axis powers. This event marked a pivotal moment that shifted the United States from a stance of isolationism to active involvement in the global conflict, demonstrating the impact of direct aggression on American foreign policy and military engagement.