What event is widely considered to have brought the United States into World War II?

The attack on Pearl Harbor on December 7, 1941, is widely considered the event that brought the United States into World War II. This surprise military strike by the Imperial Japanese Navy against the United States naval base in Hawaii resulted in significant loss of life and damage to the Pacific Fleet. It galvanized public opinion in the U.S. and led to an immediate declaration of war against Japan by Congress, with President Franklin D. Roosevelt famously describing the day of the attack as "a date which will live in infamy."

Prior events, such as the invasion of Poland and the fall of France, significantly impacted the war's dynamics and U.S. foreign policy but did not directly lead to U.S. involvement. The signing of the Atlantic Charter was a statement of mutual goals shared by the U.S. and Britain, but it was more about cooperation than active engagement in the war. Ultimately, it was Pearl Harbor that transformed the U.S. from a nation largely isolated from the conflict into an active participant in World War II.