Even while the war was proceeding, the Allies met to settle the details of the postwar world order. Their diplomatic agreements, and disagreements, reached far beyond the war’s end.

FDR and Churchill arrived at Tehran, Iran, to meet Stalin at the Tehran Conference in 1943. At this first meeting of the Big Three, the Allies planned the 1944 assault on France and agreed to divide Germany into zones of occupation after the war. They also agreed to establish a new international peacekeeping organization, the United Nations.

In February 1945, the Big Three met again in the Soviet city of Yalta. Stalin, whose troops had overrun Eastern Europe, had the most bargaining power at the Yalta Conference, leaving the other Allies with much to request but little leverage with which to force Stalin’s hand. Stalin did agree to declare war against Japan soon after Germany surrendered and approved plans for a United Nations conference in San Francisco in April 1945. Discussion of reparations, large payments that Stalin demanded from Germany that Roosevelt and Churchill opposed, was postponed.

After FDR’s death and the end of the war in Europe, Harry Truman, new British prime minister Clement Atlee, and Stalin met at the Potsdam Conference in Germany from July 17 to August 2, 1945. Little was accomplished diplomatically, as relations between the Americans and Soviets grew increasingly chilly, but the three leaders did agree to demilitarize Germany and agreed upon the concept of war crimes trials. The Potsdam Agreement divided Germany into four zones, administrated by the Soviet Union, France, Britain, and the U.S., and established joint administration of Berlin, which lay well within the Soviet zone. This arrangement proved to be a recipe for conflict in later years.

The Nuremberg Trials of Nazi war criminals began in Nuremberg, Germany, in 1945. Prosecutors charged twenty-four Germans with an assortment of crimes, including waging aggressive war, extermination of ethnic and religious groups, and murder and mistreatment of prisoners and inhabitants of occupied territories. The tribunal heard testimony and saw documentation chronicling the “crimes against humanity” perpetrated by the Nazis against European Jews and others. The Nuremberg Tribunal concluded that though not explicitly stated in international law, the instigation of aggressive war was a crime and that the defendants’ claim that they were “just following orders” was unsound because the opportunity for moral choice always existed. Twelve defendants were sentenced to death and seven others to prison sentences of varying length. After this first trial, twelve more trials were held, with about 185 Germans indicted. Only thirty-five were acquitted. The rest were sentenced to prison terms or death.

After Japan surrendered on September 2, 1945, American forces under General Douglas MacArthur occupied the country. MacArthur rigidly suppressed Japanese nationalism, held war crimes trials, and imposed democratic norms on the Japanese government. Under MacArthur’s supervision, which lasted until the end of occupation in 1952, Japan became an economically powerful democracy.

Another element of the postwar settlement in Asia was the division of Korea at the thirty-eighth parallel, an agreement reached between the Soviet Union and the U.S. shortly before the end of the war as part of the Japanese surrender. The Soviets occupied North Korea and the U.S. occupied South Korea, each supporting governments antagonistic toward each other. This antagonism would erupt in the Korean War in 1950.

In April 1945, the United Nations Conference on International Organization met in San Francisco. Delegates from fifty countries outlined their aims for global peace and collective security. In their charter, they created a General Assembly to make policy and a Security Council to settle disputes. In October 1945, the UN officially came into being, with fifty-one founding members.