Final answer:
The U.S. entered World War II due to the attack on Pearl Harbor, which shocked the nation and lead to unanimous Congressional approval of President Roosevelt's war declaration against Japan, followed by war with Germany and Italy.
Step-by-step explanation:
The United States entered World War II after being propelled from a state of isolationism by the surprise attack on Pearl Harbor by Japanese forces on December 7, 1941. This bold military strike aimed to incapacitate the U.S. Pacific Fleet and was accompanied by Japan's assault on multiple other American and allied territories throughout the Pacific. The overwhelming shock and anger felt by the American public soon turned into a unanimous call to arms, resulting in President Franklin Roosevelt's request to Congress for a declaration of war, which was approved on December 8, 1941. Shortly thereafter, with Germany and Italy declaring war on the U.S., America found itself fully engaged in the global conflict as part of the Allies, shifting the dynamics of both the Pacific and European theaters of the war.