Which event helped spur the United States into entering World War II?

Japan's attack on Pearl Harbor on December 7, 1941, was the pivotal event that led the United States to enter World War II. This surprise military strike against the U.S. naval base in Hawaii resulted in significant loss of life and damage to the Pacific fleet, effectively galvanizing public opinion in favor of entering the war. In the aftermath of the attack, President Franklin D. Roosevelt delivered his famous speech declaring December 7th a "date which will live in infamy," and Congress swiftly declared war on Japan, marking the formal entry of the United States into the conflict. The attack not only united the American populace in support of the war effort but also shifted the political landscape, culminating in a concerted military response against both Japan and, soon after, the Axis powers as a whole.