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World War II, also called Second World War, 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. The 40,000,000–50,000,000 deaths incurred in World War II make it the bloodiest conflict, as well as the largest war, in history.

World War II: Germany invading Poland World War II: Pacific Theatre of Operations World War II: Germany invading Poland Germany invading Poland, September 1, 1939. Photos.com/Thinkstock World War II: Pacific Theatre of Operations The Pacific Theatre of Operations, 1941–45. Encyclopædia Britannica, Inc. TOP QUESTIONS What was the cause of World War II? What countries fought in World War II? Who were the leaders during World War II? What were the turning points of the war? How did the war end? Along with World War I, World War II was one of the great watersheds of 20th-century geopolitical history. It resulted in the extension of the Soviet Union’s power to nations of eastern Europe, enabled a communist movement to eventually achieve power in China, and marked the decisive shift of power in the world away from the states of western Europe and toward the United States and the Soviet Union.

Churchill, Winston; Truman, Harry; Stalin, Joseph atomic bombing of Hiroshima Churchill, Winston; Truman, Harry; Stalin, Joseph British Prime Minister Winston Churchill, U.S. Pres. Harry S. Truman, and Soviet Premier Joseph Stalin meeting at Potsdam, Germany, in July 1945 to discuss the postwar order in Europe. Encyclopædia Britannica, Inc. atomic bombing of Hiroshima A gigantic mushroom cloud rising above Hiroshima, Japan, on August 6, 1945, after a U.S. aircraft dropped an atomic bomb on the city, immediately killing more than 70,000 people. U.S. Air Force photograph Axis Initiative And Allied Reaction

The outbreak of war

By the early part of 1939 the German dictator Adolf Hitler had become determined to invade and occupy Poland. Poland, for its part, had guarantees of French and British military support should it be attacked by Germany. Hitler intended to invade Poland anyway, but first he had to neutralize the possibility that the Soviet Union would resist the invasion of its western neighbour. Secret negotiations led on August 23–24 to the signing of the German-Soviet Nonaggression Pact in Moscow. In a secret protocol of this pact, the Germans and the Soviets agreed that Poland should be divided between them, with the western third of the country going to Germany and the eastern two-thirds being taken over by the U.S.S.R.

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Having achieved this cynical agreement, the other provisions of which stupefied Europe even without divulgence of the secret protocol, Hitler thought that Germany could attack Poland with no danger of Soviet or British intervention and gave orders for the invasion to start on August 26. News of the signing, on August 25, of a formal treaty of mutual assistance between Great Britain and Poland (to supersede a previous though temporary agreement) caused him to postpone the start of hostilities for a few days. He was still determined, however, to ignore the diplomatic efforts of the western powers to restrain him. Finally, at 12:40 PM on August 31, 1939, Hitler ordered hostilities against Poland to start at 4:45 the next morning. The invasion began as ordered. In response, Great Britain and France declared war on Germany on September 3, at 11:00 AM and at 5:00 PM, respectively. World War II had begun.

Britannica Premium: Serving the evolving needs of knowledge seekers. Get 30% your subscription today. Subscribe Now World War II QUICK FACTS DATE September 3, 1939 - September 2, 1945 (Anniversary in 2 days) PARTICIPANTS Australia Axis powers Canada Cuba Czechoslovakia Ethiopia Free French Luxembourg United States Allied powers KEY PEOPLE Winston Churchill Dwight D. Eisenhower Charles de Gaulle Adolf Hitler Benito Mussolini Alessandro Pertini Franklin D. Roosevelt Joseph Stalin Harry S. Truman Chesty Puller RELATED TOPICS Blitzkrieg Sherman tank Colossus Lancaster Panzer DID YOU KNOW? About 70 million total soldiers fought on behalf of the Allied or Axis countries. Switzerland, Spain, Portugal and Sweden all declared themselves neutral during WWII. Some scholars argue that the "start" of WWII was in 1937 when Japan invaded China instead of when Germany invaded Poland. Forces and resources of the European combatants, 1939 In September 1939 the Allies, namely Great Britain, France, and Poland, were together superior in industrial resources, population, and military manpower, but the German Army, or Wehrmacht, because of its armament, training, doctrine, discipline, and fighting spirit, was the most efficient and effective fighting force for its size in the world. The index of military strength in September 1939 was the number of divisions that each nation could mobilize. Against Germany’s 100 infantry divisions and six armoured divisions, France had 90 infantry divisions in metropolitan France, Great Britain had 10 infantry divisions, and Poland had 30 infantry divisions, 12 cavalry brigades, and one armoured brigade (Poland had also 30 reserve infantry divisions, but these could not be mobilized quickly). A division contained from 12,000 to 25,000 men.

Adolf Hitler Adolf Hitler Adolf Hitler reviewing troops on the Eastern Front, 1939. Heinrich Hoffmann, Munich

WORLD WAR II EVENTS keyboard_arrow_left Samuel Bak: Smoke Holocaust 1933 - 1945 default image Battle of the Atlantic September 3, 1939 - May 8, 1945 British and other Allied troops wading through the water to board ships at Dunkirk, France, 1940. Dunkirk evacuation May 26, 1940 - June 4, 1940 Smoke rising from the London Docklands after the first mass air raid on the British capital, Sept. 7, 1940. Battle of Britain June 1940 - April 1941 Archibald Percival Wavell, 1st Earl Wavell North Africa campaigns June 1940 - May 13, 1943 France, Battle of; Vichy France Vichy France July 1940 - September 1944 German bombing of London in the Battle of Britain The Blitz September 7, 1940 - May 11, 1941 German soldiers fighting in the Soviet Union as part of Operation Barbarossa, 1941. Operation Barbarossa June 22, 1941 Siege of Leningrad Siege of Leningrad September 8, 1941 - January 27, 1944 Pearl Harbor attack Pearl Harbor attack December 7, 1941 Wake Island Battle of Wake Island December 8, 1941 - December 23, 1941 U.S. troops advancing on Tarawa, Gilbert Islands, in 1943 Pacific War December 8, 1941 - September 2, 1945 Bataan Death March Bataan Death March April 9, 1942 Battle of Midway Battle of Midway June 3, 1942 - June 6, 1942 Map of eastern New Guinea from the 10th edition of Encyclopædia Britannica, c. 1902. Kokoda Track Campaign July 1942 - January 1943 Battle of Guadalcanal Battle of Guadalcanal August 1942 - February 1943 Stalingrad, Battle of Battle of Stalingrad August 22, 1942 - February 2, 1943 Warsaw Ghetto Uprising Warsaw Ghetto Uprising April 19, 1943 - May 16, 1943 default image Normandy Massacres June 1944 Normandy Invasion Normandy Invasion June 6, 1944 - July 9, 1944 Warsaw Uprising Warsaw Uprising August 1, 1944 - October 2, 1944 Cowra, New South Wales, Australia Cowra breakout August 5, 1944 Battle of Leyte Gulf Battle of Leyte Gulf October 23, 1944 - October 26, 1944 Battle of the Bulge Battle of the Bulge December 16, 1944 - January 16, 1945 Yalta Conference Yalta Conference February 4, 1945 - February 11, 1945 World War II: Corregidor Island Battle of Corregidor February 16, 1945 - March 2, 1945 Battle of Iwo Jima Battle of Iwo Jima February 19, 1945 - March 26, 1945 James H. Doolittle. Bombing of Tokyo March 9, 1945 - March 10, 1945 default image Battle for Castle Itter May 5, 1945 keyboard_arrow_right 1 2 3 4 5 6 7 8 9 10 It was the qualitative superiority of the German infantry divisions and the number of their armoured divisions that made the difference in 1939. The firepower of a German infantry division far exceeded that of a French, British, or Polish division; the standard German division included 442 machine guns, 135 mortars, 72 antitank guns, and 24 howitzers. Allied divisions had a firepower only slightly greater than that of World War I. Germany had six armoured divisions in September 1939; the Allies, though they had a large number of tanks, had no armoured divisions at that time.

WWII: D-Day in pictures BRITANNICA EXCLUSIVE ARCHIVE WWII: D-DAY IN PICTURES After a prolonged naval and aerial bombardment of German defenses on the Channel coast of France and the Low Countries, the Allied invasion of Normandy began in the early morning hours of June 6, 1944. This collection of pictures documents the historic event that created a turning point in World War II. The six armoured, or panzer, divisions of the Wehrmacht comprised some 2,400 tanks. And though Germany would subsequently expand its tank forces during the first years of the war, it was not the number of tanks that Germany had (the Allies had almost as many in September 1939) but the fact of their being organized into divisions and operated as such that was to prove decisive. In accordance with the doctrines of General Heinz Guderian, the German tanks were used in massed formations in conjunction with motorized artillery to punch holes in the enemy line and to isolate segments of the enemy, which were then surrounded and captured by motorized German infantry divisions while the tanks ranged forward to repeat the process: deep drives into enemy territory by panzer divisions were thus followed by mechanized infantry and foot soldiers. These tactics were supported by dive bombers that attacked and disrupted the enemy’s supply and communications lines and spread panic and confusion in its rear, thus further paralyzing its defensive capabilities. Mechanization was the key to the German blitzkrieg, or “lightning war,” so named because of the unprecedented speed and mobility that were its salient characteristics. Tested and well-trained in maneuvers, the German panzer divisions constituted a force with no equal in Europe.

German tanks Stuka German tanks German Pz. IV (foreground) and Pz. III (background) tanks, 1942. U.S. Army photograph Stuka German Junkers Ju 87 “Stuka” dive-bomber. UPI/Bettmann Archive