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

The 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 was conducted by the Imperial Japanese Navy against the naval base at Pearl Harbor in Hawaii. The attack resulted in significant damage to the U.S. Pacific Fleet, including the sinking of several battleships and the loss of a considerable number of American lives. In response to this assault, the United States declared war on Japan the following day, marking its formal entry into World War II.

While the other events listed, such as the German invasion of Poland, the Battle of Britain, and the Fall of France, were significant moments in the early stages of the war in Europe, they did not directly prompt U.S. involvement. The German invasion of Poland in 1939 led to the war's outbreak in Europe, and the Battle of Britain was a major conflict between Britain and Germany. The Fall of France in 1940 dramatically changed the landscape of the European theater, but it was the direct and aggressive attack on American territory at Pearl Harbor that galvanized public opinion and united the nation in support of entering the war.