World War II, 1939?45, worldwide conflict involving every major power in the world. The two sides were generally known as the Allies and the Axis.

This second global conflict resulted from the rise of totalitarian, militaristic regimes in Germany, Italy, and Japan, a phenomenon stemming in part from the Great Depression that swept over the world in the early 1930s and from the conditions created by the peace settlements (1919?20) following World War I.

After World War I, defeated Germany, disappointed Italy, and ambitious Japan were anxious to regain or increase their power; all three eventually adopted forms of dictatorship that made the state supreme and called for expansion at the expense of neighboring countries. These three countries also set themselves up as champions against Communism, thus gaining at least partial tolerance of their early actions from the more conservative groups in the Western democracies. Also important was a desire for peace on the part of the democracies, which resulted in their military unpreparedness. Finally, the League of Nations, weakened from the start by the defection of the United States, was unable to promote disarmament; moreover, the long economic depression sharpened national rivalries, increased fear and distrust, and made the masses susceptible to the promises of demagogues.

The failure of the League to stop the Second Sino-Japanese War in 1931 was followed by a rising crescendo of treaty violations and acts of aggression. Adolf Hitler, when he rose to power (1933) in Germany, recreated the German army and prepared it for a war of conquest; in 1936 he remilitarized the Rhineland. Benito Mussolini conquered (1935?36) Ethiopia for Italy; and from 1936 to 1939 the Spanish civil war raged, with Germany and Italy helping the fascist forces of Francisco Franco to victory. In Mar., 1938, Germany annexed Austria, and in Sept., 1938, the British and French policy of appeasement toward the Axis reached its height with the sacrifice of much of Czechoslovakia to Germany in the Munich Pact.

When Germany occupied (Mar., 1939) all of Czechoslovakia, and when Italy seized (Apr., 1939) Albania, Great Britain and France abandoned their policy of appeasement and set about creating an ?antiaggression? front, which included alliances with Turkey, Greece, Romania, and Poland, and speeding rearmament. Germany and Italy signed (May, 1939) a full military alliance, and after the Soviet-German nonaggression pact (Aug., 1939) removed German fear of a possible two-front war, Germany was ready to launch an attack on Poland.

World War II began on Sept. 1, 1939, when Germany, without a declaration of war, invaded Poland. Britain and France declared war on Germany on Sept. 3, and all the members of the Commonwealth of Nations, except Ireland, rapidly followed suit. The fighting in Poland was brief. The German blitzkrieg, or lightning war, with its use of new techniques of mechanized and air warfare, crushed the Polish defenses, and the conquest was almost complete when Soviet forces entered (Sept. 17) E Poland. While this campaign ended with the partition of Poland and while the USSR defeated Finland in the Finnish-Russian War (1939?40), the British and the French spent an inactive winter behind the Maginot Line, content with blockading Germany by sea.

The inactive period ended with the surprise invasion (Apr. 9, 1940) of Denmark and Norway by the Germans. Denmark offered no resistance; Norway was conquered by June 9. On May 10, German forces overran Luxembourg and invaded the Netherlands and Belgium; on May 13 they outflanked the Maginot Line. Their armored columns raced to the English Channel and cut off Flanders, and Allied forces were evacuated from Dunkirk (May 26?June 4). General Weygand had replaced General Gamelin as supreme Allied commander, but was unable to stop the Allied debacle in the ?battle of France.? On June 22, France signed an armistice with Germany, followed by an armistice with Italy, which had entered the war on June 10. The Vichy government was set up in France under Marshal P?tain. Britain, the only remaining Allied power, resisted, under the inspiring leadership of Winston Churchill, the German attempt to bomb it into submission.

While Germany was receiving its first setback in the Battle of Britain, fought entirely in the air, the theater of war was widened by the Italian attack on the British in North Africa of Greece, and by German submarine warfare in the Atlantic Ocean. Hungary, Romania, and Bulgaria joined the Axis late in 1940, but Yugoslavia resisted German pressure, and on Apr. 6, 1941, Germany launched attacks on Yugoslavia and Greece and won rapid victories. In May, Crete fell.

Great Britain gained a new ally on June 22, 1941, when Germany (joined by Italy, Romania, Hungary, Slovakia, and Finland), invaded the Soviet Union. By Dec., 1941, German mechanized divisions had destroyed a substantial part of the Soviet army and had overrun much of European Russia. However, the harsh Russian winter halted the German sweep, and the drive on Moscow was foiled by a Soviet counteroffensive.

Though determined to maintain its neutrality, the United States was gradually drawn closer to the war by the force of events. To save Britain from collapse the Congress voted lend-lease aid early in 1941. In Aug., 1941, President Franklin Delano Roosevelt met Churchill on the high seas, and together they formulated the Atlantic Charter as a general statement of democratic aims. To establish bases to protect its shipping from attacks by German submarines, the United States occupied (Apr., 1941) Greenland and later shared in the occupation of Iceland; despite repeated warnings, the attacks continued. Relations with Germany became increasingly strained, and the aggressive acts of Japan in China, Indochina, and Thailand provoked protests from the United States.

Efforts to reach a peaceful settlement were ended on Dec. 7, 1941, when Japan without warning attacked Pearl Harbor, the Philippines, and Malaya. War was declared (Dec. 8) on Japan by the United States, the Commonwealth of Nations (except Ireland), and the Netherlands. Within a few days Germany and Italy declared war on the United States.

The first phase of the war in the Pacific was disastrous for the Allies. Japan swiftly conquered the Philippines (where strong resistance ended at Corregidor), Malaya, Burma (Myanmar), Netherlands East Indies (Indonesia), and many Pacific islands; destroyed an Allied fleet in the Java Sea; and reached, by mid-1942, its furthest points of advance in the Aleutian Islands and New Guinea.

Australia became the chief Allied base for the countermoves against Japan, directed by Gen. Douglas MacArthur, Admiral Nimitz, and Admiral Halsey. The first Allied naval successes against Japan were scored in the battles of the Coral Sea and Midway, where U.S. bombers knocked out the major part of Japan's carrier fleet and forced Japan into retreat. Midway was the first decisive blow against the Axis by Allied forces. On land the Allies took the offensive in New Guinea and landed (Aug. 7, 1942) on Guadalcanal in the Solomon Islands.

Despite the slightly improved position in the Pacific, the late summer of 1942 was perhaps the darkest period of the war for the Allies. In North Africa, the Axis forces under Field Marshal Rommel were sweeping into Egypt; in Russia, they had penetrated the Caucasus and launched a gigantic offensive against Stalingrad. In the Atlantic, even to the shores of the United States and in the Gulf of Mexico, German submarines were sinking Allied shipping at an unprecedented rate.

Yet the Axis war machine showed signs of wear, while the United States was merely beginning to realize its potential, and Russia had huge reserves and was receiving U.S. lend-lease aid through Iran and the port of Murmansk. The major blow, however, was leveled at the Axis by Britain, when General Montgomery routed Rommel at Alamein in North Africa (Oct., 1942). This was followed by the American invasion of Algeria (Nov. 8, 1942); the Americans and British were joined by Free French forces of General de Gaulle and by regular French forces that had passed to the Allies after the surrender of Admiral Darlan. After heavy fighting in Tunisia, North Africa was cleared of Axis forces by May 12, 1943.

Meantime, in the Soviet stand at Stalingrad and counteroffensive resulted in the surrender (Feb. 2, 1943) of the German 6th Army, followed by nearly uninterrupted Russian advances. In the Mediterranean, the Allies followed up their African victory by the conquest of Sicily (July?Aug., 1943) and the invasion of Italy, which surrendered on Sept. 8. However, the German army in Italy fought bloody rearguard actions, and Rome fell (June 4, 1944) only after the battles of Monte Cassino and Anzio. In the Atlantic, the submarine threat was virtually ended by the summer of 1944. Throughout German-occupied Europe, underground forces, largely supplied by the Allies, began to wage war against their oppressors.

The Allies, who had signed (Jan. 1, 1942) the United Nations declaration, were drawn closer together militarily by the Casablanca Conference, at which they pledged to continue the war until the unconditional surrender of the Axis, and by the Moscow Conferences, the Quebec Conference, the Cairo Conference, and the Tehran Conference. The invasion of German-held France was decided upon, and Gen. Dwight D. Eisenhower was put in charge of the operation.

By the beginning of 1944 air warfare had turned overwhelmingly in favor of the Allies, who wrought unprecedented destruction on many German cities and on transport and industries throughout German-held Europe. This air offensive prepared the way for the landing (June 6, 1944) of the Allies in N France and a secondary landing (Aug. 15) in S France. After heavy fighting in Normandy, Allied armored divisions raced to the Rhine, clearing most of France and Belgium of German forces by Oct., 1944. The use of V-1 and V-2 rockets by the Germans proved as futile an effort as their counteroffensive in Belgium under General von Rundstedt.

On the Eastern Front Soviet armies swept (1944) through the Baltic States, E Poland, Belorussia, and Ukraine and forced the capitulation of Romania (Aug. 23), Finland (Sept. 4), and Bulgaria (Sept. 10). Having evacuated the Balkan Peninsula, the Germans resisted in Hungary until Feb., 1945, but Germany itself was pressed. The Russians entered East Prussia and Czechoslovakia (Jan., 1945) and took E Germany to the Oder.

On Mar. 7 the Western Allies?whose chief commanders in the field were Omar N. Bradley and Montgomery?crossed the Rhine after having smashed through the strongly fortified Siegfried Line and overran W Germany. German collapse came after the meeting (Apr. 25) of the Western and Russian armies at Torgau in Saxony, and after Hitler's death amid the ruins of Berlin, which was falling to the Russians under marshals Zhukov and Konev. The unconditional surrender of Germany was signed at Reims on May 7 and ratified at Berlin on May 8.

After the completion of the campaigns in the Solomon Islands (late 1943) and New Guinea (1944), the Allied advance moved inexorably, in two lines that converged on Japan, through scattered island groups?the Philippines, the Mariana Islands, Okinawa, and Iwo Jima. Japan, with most of its navy sunk, staggered beneath these blows. At the Yalta Conference, the USSR secretly promised its aid against Japan, which still refused to surrender even after the Allied appeal made at the Potsdam Conference. On Aug. 6, 1945, the United States first used the atomic bomb and devastated Hiroshima; on Aug. 9, the second bomb was dropped on Nagasaki. The USSR had already invaded Manchuria. On Aug. 14, Japan announced its surrender, formally signed aboard the U.S. battleship Missouri in Tokyo Bay on Sept. 2.

Although hostilities came to an end in Sept., 1945, a new world crisis caused by the postwar conflict between the USSR and the United States?the two chief powers to emerge from the war?made settlement difficult. By Mar., 1950, peace treaties had been signed with Italy, Romania, Hungary, Bulgaria, and Finland; in 1951, the Allies (except the USSR) signed a treaty with Japan, and, in 1955, Austria was restored to sovereignty. Germany, however, remained divided?first between the Western powers and the USSR, then (until 1990) into two German nations.

Despite the birth of the United Nations, the world remained politically unstable and only slowly recovered from the incalculable physical and moral devastation wrought by the largest and most costly war in history. Soldiers and civilians both had suffered in bombings that had wiped out entire cities. Modern methods of warfare?together with the attempt of Germany to exterminate entire religious and ethnic groups (particularly the Jews)?famines, and epidemics, had brought death to tens of millions and made as many more homeless. The suffering and degradation of the war's victims were of proportions that passed the understanding of those who had been spared. The conventions of warfare had been violated on a large scale, and warfare itself was revolutionized by the development and use of nuclear weapons.

Political consequences included the reduction of Britain and France to powers of lesser rank, the emergence of the Common Market, the independence of many former colonies in Asia and Africa, and, perhaps most important, the beginning of the cold war between the Western powers and the Communist-bloc nations.