What U.S. response did the Pearl Harbor attack directly trigger?

The Pearl Harbor attack directly triggered a U.S. declaration of war on Japan.

President Franklin D. Roosevelt asked Congress for a declaration on December 8, 1941, one day after the attack. Congress approved it, and Roosevelt signed the measure, formally bringing the United States into the Pacific War against Japan. The vote in the House of Representatives was 388 to 1, with Representative Jeannette Rankin casting the lone opposing vote.

The declaration did not immediately constitute a U.S. declaration of war against Germany or Italy. Germany and Italy declared war on the United States on December 11, 1941, after which Congress issued declarations against both countries. The United States then became a formal combatant in the broader Second World War.

The attack transformed American public opinion and ended the country’s practical neutrality. Before Pearl Harbor, the United States had supported countries fighting the Axis through measures such as Lend-Lease, but it had not formally entered the war.