What event marked the escalation of US involvement in World War II?

The bombing of Pearl Harbor on December 7, 1941, is widely recognized as the event that marked the escalation of U.S. involvement in World War II. This surprise military attack by the Imperial Japanese Navy resulted in significant loss of life and damage to the Pacific Fleet stationed at Pearl Harbor, Hawaii. In response to the attack, the United States declared war on Japan the following day, December 8, 1941, thus officially entering the conflict.

The magnitude and shock of the attack galvanized public opinion and unified the country in support of the war effort, which ultimately led to a full-scale military mobilization and a commitment to Allied forces in Europe and the Pacific. Prior to this event, U.S. involvement had been more limited, focused on providing material support to allies rather than direct combat engagement.

In contrast, while Japan's invasion of China and the Tripartite Pact contributed to the tensions leading up to the war, they did not directly involve the U.S. in fighting, which intensified only after Pearl Harbor. The liberation of France, occurring several years later, represented a stage in the conflict rather than a catalyst for U.S. entry.