Initially, the Soviet Union was on the side of the Axis Powers, but Adolf Hitler’s decision to invade the Soviet Union in 1941 caused Soviet leader Joseph Stalin to align with the Allies. As the Nazis quickly penetrated deep into Soviet territory, victory seemed all but assured. During this time, they established a plan known as the “Final Solution” to systematically kill all Jews in the territories under their control. Having already established concentration camps in which to imprison political enemies and other undesirables, the Nazi regime created its first extermination camp at Auschwitz in Poland in 1941, building gas chambers to more efficiently kill prisoners. This was the beginning of the Holocaust.

The invasion of the Soviet Union stalled, however, when an early winter combined with the stress of maintaining long supply lines and a forceful Soviet counterattack brought the advance to a halt. Germany was now facing a protracted war on two fronts. The entrance of the United States to the war following Japan’s surprise attack on the American naval base at Pearl Harbor in Hawaii on December 7, 1941, brought considerable resources and manpower to the side of the Allies from 1941-1945. President Franklin D. Roosevelt called the fateful event "a day that will live in infamy" as he appealed to the American people to enter the conflict. Congress authorized a declaration of war shortly after the attack. The U.S. Navy, who had been surprised at Pearl Harbor, would later engage with Japan in several key battles in the Pacific region, including the decisive Battle of Midway in 1942.

On the U.S. home front, American women accepted jobs in factories that supplied the military with war materiel and rations, inspiring the iconic wartime image of "Rosie the Riveter." Men joined the ranks of the armed forces, including the U.S. Army and the Air Force. Because of the Pearl Harbor attack, Franklin D. Roosevelt also authorized the detainment of Japanese Americans in internment camps, including Manzanar, for the duration of their time in the war. The discriminatory wartime treatment of Japanese Americans was regarded by many historians as a shameful act in U.S. history.

After the United States joined the war, the Axis powers began to lose key battles in the European theater, including the Battle of Stalingrad in 1942–1943 and the 1943 invasion of Italy that forced Italy to surrender. The war was the first to cover land, sea, and air to such a vast degree with the German Luftwaffe fighting in the sky and German U-boats (submarines) trolling the seas. Bombing raids occurred over several British cities, including London. The most critical military action during the conflict, however, was the massive Allied invasion of France known as D-Day, or Operation Neptune, which launched on June 6, 1944. Also known as the Normandy invasion, it was the largest seaborne invasion in history, as U.S., British, and Canadian servicemen landed on five separate beachheads in Normandy, France, to break through German defenses. The Allies slowly advanced on the western front for nearly a year more, while the Soviet Union pushed German forces back in the east. Following the Allied capture of Berlin, Germany unconditionally surrendered on May 8, 1945. It would have to answer for its war crimes in the internationally conducted Nuremberg Trials.

Despite the loss of its two primary allies, Japan refused to surrender. Hoping to quickly end the war, the United States decided to make use of a new secret weapon it had developed during the war: the atomic bomb. The destruction of the Japanese cities of Hiroshima and Nagasaki pushed Japan to surrender on August 15, 1945. WWII was over.

In the aftermath of the war, Europe was divided into a US-led Western bloc and a Soviet-led Eastern bloc to begin the long process of reconstruction. It was not long before these two superpowers became intense military and political rivals, which would lead to the beginning of the Cold War and the nuclear arms race. U.S. servicemen who fought in World War II were commemorated with the establishment of a number of museums and memorial sites, including burial grounds, under the administration of the American Battle Monuments Commission.