Which event prompted the United States to enter World War II?

The attack on Pearl Harbor is widely recognized as the immediate catalyst that prompted the United States to formally enter World War II. On December 7, 1941, Japan launched a surprise military strike against the U.S. naval base in Pearl Harbor, Hawaii. This attack resulted in significant loss of life and damage to the Pacific Fleet, galvanizing public opinion in the United States to support entering the war.

Prior to the attack, the U.S. had maintained a policy of neutrality and was largely uninvolved in European conflicts, although there was growing sympathy for the Allies and increasing economic aid. However, the scale and suddenness of the Pearl Harbor attack shifted national sentiment dramatically, leading to Congress declaring war on Japan the following day, thereby marking the official entry of the United States into World War II.

While the invasion of Poland by Germany in 1939 is often seen as the event that sparked the beginning of the war in Europe, it did not compel the U.S. to join the conflict until the direct attack on American soil made engagement unavoidable. The fall of France and the Battle of Britain demonstrated the ongoing struggles in Europe but did not trigger U.S. involvement as directly as the attack on Pearl Harbor did.