What event led to the United States entering World War II?

The event that led to the United States entering World War II was the attack on Pearl Harbor. This surprise military strike by the Imperial Japanese Navy on December 7, 1941, aimed to incapacitate the U.S. Pacific Fleet and prevent it from interfering with Japanese military actions in Southeast Asia. The attack resulted in significant loss of life and damage to the U.S. naval fleet, provoking public outrage and unifying the American public in support of entering the war. Consequently, the United States declared war on Japan the following day, effectively entering World War II.

The other options represent significant events but did not directly lead to U.S. involvement in the war. The signing of the Treaty of Versailles marked the end of World War I and laid some of the groundwork for future global conflicts but did not trigger American entry into World War II. The invasion of Germany relates to the European theater of the war but occurred after the U.S. had already entered the conflict. Similarly, while the Battle of Britain was a crucial series of air battles between Britain and Germany, it did not instigate U.S. participation in the war. The attack on Pearl Harbor was the decisive factor that galvanized the United States into action against the Axis powers.