Florida US History EOC Practice Exam 2025 - Free EOC Practice Questions and Study Guide

The event that occurred on December 7, 1941, that significantly changed U.S. involvement in World War II was the attack on Pearl Harbor by the Japanese. This surprise military strike led to substantial loss of life and damage to the U.S. Pacific Fleet stationed at Pearl Harbor in Hawaii. As a result of the attack, which was perceived as an unprovoked act of aggression, the United States declared war on Japan the following day, December 8, 1941. This declaration marked the official entry of the U.S. into World War II, shifting its role from neutrality to active involvement in the conflict on both the Pacific and European fronts.

Other events listed, such as the signing of the Lend-Lease Act, occurred prior to this and were instrumental in supporting the Allies, but did not directly involve the United States in combat. The invasion of Poland occurred in 1939 and was a catalyst for the war in Europe, while the dropping of atomic bombs on Hiroshima took place much later, in 1945, as a conclusion to the war. Thus, the attack on Pearl Harbor stands out as the pivotal moment that galvanized U.S. military action in World War II.