There were several incidents that led to the United States’ decision to enter World War II, but the most significant one was the Japanese attack on Pearl Harbor on December 7, 1941.
Prior to this attack, the United States had adopted a policy of neutrality and had been providing military aid to the Allied Powers, particularly Great Britain and the Soviet Union. However, President Franklin D. Roosevelt was facing opposition from isolationists who were opposed to any involvement in the war.
The attack on Pearl Harbor, which resulted in the deaths of over 2,400 Americans and the destruction of much of the U.S. Pacific Fleet, galvanized the American public and led to a declaration of war against Japan the following day. Germany and Italy, which were allied with Japan, then declared war on the United States, and the U.S. entered the war on the side of the Allies.