What incident directly prompted the United States to enter World War II?

The attack on Pearl Harbor on December 7, 1941, was the pivotal incident that led to the United States’ entry into World War II. This surprise military strike by the Imperial Japanese Navy resulted in significant damage to the U.S. Pacific Fleet, destroying battleships, aircraft, and killing over 2,400 Americans. The attack galvanized American public opinion, which had been largely isolationist, and led President Franklin D. Roosevelt to ask Congress for a declaration of war against Japan the following day. This event marked a dramatic turning point in U.S. foreign policy, as the nation moved from neutrality to active involvement in the conflict.

The other options represent significant events that had an influence on the war or international relations, but they did not directly result in the United States declaring war. The bombing of Hiroshima occurred later in the war, during the conflict with Japan. The invasion of France was a critical event in Europe, but it did not directly provoke American involvement. The sinking of the Lusitania was a notable incident in World War I that influenced American opinion against Germany but occurred several decades before the United States entered World War II.