World War II 1939-45 also called Second World War, a conflict that involved virtually every part of the world during the years 1939-45. The principal belligerents were the Axis powers--Germany, Italy, and Japan--and the Allies--France, Great Britain, the United States, the Soviet Union, and, to a lesser extent, China. The war was in many respects a continuation, after an uneasy 20-year hiatus, of the disputes left unsettled by World War I.

German bitterness over their defeat in World War I and the harsh terms of the Treaty of Versailles, together with the social unrest and political instability that beset the Weimar Republic, resulted in the coming to power of Adolf Hitler, leader of the intensely nationalistic and anti-Semitic National Socialist (Nazi) Party. Given dictatorial powers in 1933, Hitler began the secret rearmament of Germany almost immediately. Playing on the reluctance of other European powers to actively oppose him, he ordered the military occupation of the Rhineland, in contravention of the Treaty of Versailles, in March 1936. Later that year Benito Mussolini, the fascist dictator of Italy, who had already embarked on aggression in Ethiopia, declared a Rome-Berlin "axis"; the next year Italy joined the 1936 Anti-Comintern Pact between Germany and Japan. Germany and Italy both intervened, in the name of anticommunism, in the Spanish Civil War from 1936.

In March 1938 Hitler sent German troops to occupy Austria, which was promptly incorporated into Germany. By a combination of external and internal pressures he succeeded in annexing or neutralizing all of what had been Czechoslovakia by March 1939. In April 1939 Italy annexed Albania. On September 1, secure behind the new German-Soviet Nonaggression Pact that had stunned the world in August, Hitler began an invasion of Poland. Great Britain and France declared war on Germany two days later. By the end of 1939 Germany and the Soviet Union had divided Poland between themselves, and the Soviets had occupied Estonia, Latvia, and Lithuania and had attacked Finland, which they finally defeated in March 1940. For some months Germany's main activities were at sea, including an effective submarine campaign against merchant shipping bound for Britain. In April 1940 Germany occupied several Norwegian ports and all of Denmark. On May 10 the major German offensive in the west began with a lightning sweep through The Netherlands and Belgium into France; by June 22 three-fifths of France, including Paris, was occupied, and the rest had become a neutral state with its government at Vichy. During August-September the German Luftwaffe (Air Force) launched massive bombing raids on Great Britain in an attempt to soften it up for a cross-Channel invasion. The Battle of Britain was won by the Royal Air Force, however, and Hitler postponed the invasion indefinitely.

Following Italy's abortive invasion of Greece in November 1940, Hitler drew Hungary, Romania, and Slovakia into the Axis; Bulgaria joined in March 1941. In April Germany attacked Yugoslavia and Greece, both of which were overrun by the end of the month. In June Hitler abandoned the Nonaggression Pact of 1939 and launched a massive surprise invasion of the Soviet Union. German armoured units drove deep into Soviet territory and at one point reached the outskirts of Moscow before Soviet counterattacks and winter weather slowed the offensive to a halt.

Japan, the other Axis member, had meanwhile been tiring of its long, unproductive war in China and decided to take advantage of the situation in Europe to seize European colonial holdings in the Far East. To cripple what it foresaw would be its main opponent in a Pacific war of aggrandizement, Japan attacked United States installations at Pearl Harbor, Hawaii, and the Philippines on Dec. 7-8, 1941. Within days the United States was at war with all the Axis powers. Japan swiftly invaded and occupied the Philippines, most of Southeast Asia and Burma (Myanmar), the Netherlands East Indies (now Indonesia), and many Pacific Ocean islands. Despite the enormous initial advantage gained by its sudden offensives, Japan lost the crucial sea battle of Midway in June 1942. The American strategy in the Pacific was to use naval and amphibious forces to advance up the chains of islands toward Japan while smaller land forces cooperated with Chinese and British efforts on the Asian mainland.

In North Africa the British, who in 1940-41 had defeated much larger Italian forces, were locked in a seesaw battle with the German Afrika Korps. In November 1942 the first Allied offensive began with U.S. and British landings in North Africa. German forces were gradually squeezed into Tunisia and were finally eliminated in May 1943. In July Allied troops from North Africa landed in Sicily and thence invaded Italy in September. The fascist government was overthrown, and in October, Italy joined the Allies; fighting against German troops continued in Italy for the rest of the war.

After a bitterly opposed and finally unsuccessful attack on Stalingrad (August 1942-February 1943), German forces in the Soviet Union lost momentum, and, as the Red Army continued to draw on its huge manpower reserves, it began during 1943 to push the Germans back from the western portions of the Soviet Union. Germany was meanwhile preparing for an expected Allied invasion of western Europe. The invasion came on June 6, 1944--D-Day--on the beaches of Normandy in northern France, where 156,000 British, Canadian, and U.S. troops under the command of the U.S. general Dwight D. Eisenhower were landed. With command of the air the Allies quickly consolidated their foothold and began the advance eastward that ended in the occupation of the German homeland in March-April 1945. Meanwhile, the Soviet forces in 1944 had pushed the Germans completely out of the Soviet Union and had advanced into Poland, Czechoslovakia, Hungary, and Romania. In early 1945 they occupied the eastern one-third of Germany. At the climax of the German collapse, with Berlin encircled by Soviet troops, Hitler committed suicide on April 30; on May 8 the surrender of all German forces was signed.

In the Pacific the "island-hopping" strategy of the U.S. general Douglas MacArthur led to the Allied invasion of the Philippines by October 1944. The naval battle in Leyte Gulf that followed all but eliminated the Japanese navy. The capture, after bitter fighting, of the islands of Iwo Jima and Okinawa in March and June of 1945 opened the way for both the heavy strategic bombing of Japan itself and a possible invasion. The war in the Pacific came to a sudden and dramatic close after the atomic bombing of Hiroshima and Nagasaki on Aug. 6 and 9, 1945; Japan's formal surrender was signed on September 2.