What event brought the United States into World War II?

The attack on Pearl Harbor is the event that directly led to the United States entering World War II. On December 7, 1941, the Japanese launched a surprise military strike against the U.S. naval base at Pearl Harbor in Hawaii, which resulted in significant loss of life and damage to the Pacific Fleet. This attack galvanized public opinion in the United States, which had been largely isolationist prior to this point, and prompted Congress to declare war on Japan the following day, effectively bringing the U.S. into the global conflict.

The invasion of Poland, while significant in triggering World War II in Europe in 1939, did not involve the United States directly at that time, as the U.S. was not in the war then. The bombing of Hiroshima occurred in August 1945 and was an event that took place after the U.S. had already entered the war, serving as part of the conclusion to the conflict rather than its initiation. The signing of the Treaty of Versailles in 1919 marked the end of World War I, which is historically separate from the events that led to World War II.