The phrase "a day that will live in infamy" refers to December 7, 1941, when Japan launched a surprise attack on the United States naval base at Pearl Harbor, Hawaii. This attack resulted in significant loss of life and damage to the Pacific Fleet, prompting the United States to formally enter World War II. President Franklin D. Roosevelt delivered a poignant speech to Congress the following day, condemning the attack and rallying the nation for war. The event not only marked a turning point in the war but also drastically changed the course of American foreign policy and its role on the global stage.