World War II

1939–1945

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WRITTEN BY

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John Graham Royde-Smith See All Contributors

Associate Editor, History, Encyclopædia Britannica, London.

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LAST UPDATED: Aug 27, 2020 See Article History

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Alternative Titles: Second World War, WWII

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.

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World War II: Germany invading PolandGermany invading Poland, September 1, 1939.Photos.com/Thinkstock

World War II: Pacific Theatre of OperationsThe Pacific Theatre of Operations, 1941–45.Encyclopædia Britannica, Inc.

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What was the cause of World War II?

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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, JosephBritish 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.

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atomic bombing of HiroshimaA 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

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Axis Initiative And The Allies

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The outbreak of war

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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.

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World War II

QUICK FACTS

DATE

• September 3, 1939 - September 2, 1945
  (Anniversary in 5 days)

PARTICIPANTS

• Axis powers

• Belgium

• China

• Cuba

• Czechoslovakia

• France

• Free French

• India

• Poland

• 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.