At 7:48 AM on December 7, 1941, Japanese aircraft descended on Pearl Harbor, Hawaii, beginning a 110-minute assault that would kill 2,403 Americans and wound 1,178 others. The attack destroyed 188 American aircraft and sank or damaged 21 ships, including the USS Arizona (BB-39), whose explosion killed 1,177 of her 1,512 crew—the largest single-ship loss in American naval history. Two waves of 353 Japanese aircraft, launched from six aircraft carriers 200 miles north of Oahu, achieved strategic surprise and tactical devastation. By December 8, as the full implications became clear to American military and political leadership, the nation was transformed.

President Franklin Delano Roosevelt, who had been informed of the attack late on December 7 in the evening, appeared before Congress at 12:30 PM on December 8, 1941, to deliver what history would call the "Day of Infamy" speech. His words were direct and powerful: "Yesterday, December 7, 1941—a date which will live in infamy—the United States of America was suddenly and deliberately attacked by naval and air forces of the Empire of Japan." Roosevelt asked for a declaration of war. Congress acted with remarkable speed and overwhelming consensus. The Senate voted 82–0 for war; the House voted 388–1, with only Republican Representative Jeannette Rankin of Montana dissenting, as she had done in the vote for war in 1917. The United States had entered World War II.

The day's fighting was not limited to Pearl Harbor. On December 8, Japanese forces attacked American installations across the Pacific. The Philippines, where American General Douglas MacArthur commanded approximately 30,000 American troops and 100,000 Filipino soldiers, came under immediate assault. Japanese aircraft attacked airfields at Clark Field and Nichols Field, destroying American combat aircraft on the ground. Wake Island, home to fewer than 500 American military personnel, came under air attack. Guam, defended by only 153 American service members, fell to Japanese assault within 24 hours. Midway Island received air attacks. Across the entire Pacific, Japanese military operations proceeded according to long-prepared plans. The attacks were coordinated, overwhelming, and designed to achieve Japanese dominance of the Pacific before American industrial power could be mobilized.

On the American mainland on December 8, panic and mobilization competed for the public consciousness. In Los Angeles, rumors of an imminent Japanese air attack sparked a civilian air raid alert on December 8. Military and civil defense authorities issued warnings; residents sheltered; searchlights swept the night sky; air raid wardens directed blackout procedures. No Japanese aircraft appeared, but the alert demonstrated the psychological shock of the attack and the speed with which America's civilian population was forced to contemplate military threat. On the East Coast, the Army Chief of Staff, General George C. Marshall, issued emergency orders for coastal defense and mobilization of reserves. Every military installation in the United States was placed on high alert.

The salvage of the USS Nevada, the only American battleship to get underway during the initial attack, continued on December 8. Hit by a bomb and a torpedo, and threatened with sinking in the narrow channel and blocking the harbor, the Nevada's commanding officer, Captain Francis J. Thomas, and his crew made a desperate decision: beach the ship to prevent her from sinking and blocking the harbor entrance. The Nevada was intentionally grounded in shallow water on the west side of Ford Island. Salvage teams, recognizing the ship's value and not yet knowing the course of the war, immediately began working to prevent her complete loss. She would be refloated, repaired, and return to combat service. On December 8, however, the outcome remained uncertain; the ship lay smoking in the harbor, a visible symbol of American military vulnerability.

The diplomatic dimension unfolded with equal speed. Japan's allies—Nazi Germany and Fascist Italy—faced a choice. Germany declared war on the United States on December 11; Italy followed on the same day. Hitler, convinced that an American entry into the European war would eventually seal Germany's defeat but believing that Japan's entry would tie down American resources in the Pacific, chose to honor Germany's alliance commitment. The global conflict, already spanning Europe, Africa, and Asia, now became truly universal in scope.

Great Britain's response was immediate. Prime Minister Winston Churchill, who had been seeking to bring America into the war against Nazi Germany for over a year, moved quickly to coordinate with the United States. On December 7 evening, Churchill telephoned Roosevelt. The two leaders, who had been conducting secret correspondence and military staff talks for months, now became formal military allies. Churchill recognized immediately that American entry, despite its terrible beginning, transformed the strategic picture entirely. American industrial capacity and manpower could break the deadlock that had settled over European warfare.

December 8, 1941, marked the true beginning of American participation in World War II. The shock of the attack would gradually transform into determination and mobilization. American industrial production, already ramping up through Lend-Lease assistance to Britain and the Soviet Union, would now focus on arming American forces. Millions of young American men would be drafted. Civilians would be rationed and asked to contribute to the war effort. But on December 8, grief, shock, and rage predominated. The nation's military had been defeated in a single day. America faced the staggering task of rebuilding its Pacific Fleet and preparing to fight a two-front global war.