The correct answer is The attack on Pearl Harbor.
The United States military base in Hawaii was attacked by the Japanese military on Sunday, December 7th 1941. This attack by the Japanese military killed US military personnel, causing an outraged response by the American government and citizens alike. This event became a national tragedy and prompted the United States to enter World War II just one day after the attack. This attack was used in multiple pieces of propaganda in order to encourage young men to enlist in the US military.