Soon after the United States entered the war, the western Allies decided that their essential military effort was to be concentrated in Europe, where the core of enemy power lay, while the Pacific theater was to be secondary.
On November 7, 1942, an American army landed in French North Africa, and after hard-fought battles, inflicted severe defeats on Italian and German armies. The year 1942 was also the turning point on the Eastern Front, where the Soviet Union, suffering immense losses, stopped the Nazi invasion at the gates of Leningrad and Moscow, and defeated the German forces at Stalingrad.

In July 1943 British and American forces invaded Sicily, and by late summer the southern shore of the Mediterranean was cleared of Fascist forces. Allied forces landed on the Italian mainland, and although the Italian government accepted unconditional surrender, fighting against Nazi forces in Italy was bitter and protracted. Rome was not liberated until June 4, 1944. While battles were still raging in Italy, Allied forces made devastating air raids on German railroads, factories and weapon emplacements, including German oil supplies at Ploesti in Romania.

Late in 1943 the Allies, after much debate over strategy, decided to open a Western front to force the Germans to divert far larger forces from the Russian front. U.S. General Dwight D. Eisenhower was appointed Supreme Commander of Allied Forces in Europe. After immense preparations, on June 6, 1944, the first contingents of a U.S., British and Canadian invasion army, protected by a greatly superior air force, landed on the beaches of Normandy in northern France.
The Allied armies began to move across France toward Germany. On August 25 Paris was liberated. At the borders of Germany, the Allies were delayed by stubborn counteraction, but by February and March 1945, troops advanced into Germany from the west, and German armies fell before the Russians in the east. On May 8 all that remained of the Third Reich surrendered its land, sea and air forces.

The war in the Pacific continued after Germany's surrender, and the final battles there were among the hardest fought.
The heads of the U.S., British and Soviet governments met at Potsdam, a suburb outside Berlin, from July 17, to August 2, 1945, to discuss operations against Japan, the peace settlement in Europe, and a policy for the future of Germany.

The day before the Potsdam Conference began, an atomic bomb was exploded at Alamogordo, New Mexico, the culmination of three years of intensive research in laboratories across the United States in what was known as the Manhattan Project. President Truman, calculating that an atomic bomb might be used to gain Japan's surrender more quickly and with fewer casualties than an invasion of the mainland, ordered the bomb be used if the Japanese did not surrender by August 3. The Allies issued the Potsdam Declaration on July 26, promising that Japan would neither be destroyed nor enslaved if it surrendered; if Japan did not, however, it would meet "utter destruction."

On August 6, a U.S. plane, the Enola Gay, dropped an atomic bomb on the city of Hiroshima. On August 8, a second atomic bomb was dropped, this time on Nagasaki. Americans were relieved that the bomb hastened the end of the war; the realization of its awesome destructiveness would come later. On August 14, Japan agreed to the terms set at Potsdam. On September 2, 1945, Japan formally surrendered.