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Almost every country in the world participated in World War II. Most were neutral at the beginning, but only a relative few nations remained neutral to the end. The Second World War pitted two alliances against each other, the Axis powers and the Allied powers.
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- Overview
- The outbreak of war
World War II began in Europe on September 1, 1939, when Germany invaded Poland. Great Britain and France responded by declaring war on Germany on September 3. The war between the U.S.S.R. and Germany began on June 22, 1941, with Operation Barbarossa, the German invasion of the Soviet Union. The war in the Pacific began on December 7/8, 1941, when Japan attacked the American naval base at Pearl Harbor and other American, Dutch, and British military installations throughout Asia.
Read more below: Axis initiative and Allied reaction: The outbreak of war
Pacific War
Read more about the war in the Pacific.
What countries fought in World War II?
The main combatants 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).
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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Pop Quiz: 17 Things to Know About World War II
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.
World War II is generally considered to have begun on 1 September 1939, when Nazi Germany, under Adolf Hitler, invaded Poland. The United Kingdom and France declared war on Germany on 3 September.
Oct 29, 2009 · Learn about the causes, course and consequences of World War II, the largest and deadliest conflict in human history. Find out how many countries were involved in the war and what roles they played.
- 2 min
Oct 5, 2021 · World War II was a conflict that involved virtually every part of the world during 1939–45. The main combatants were the Axis powers (Germany, Italy, and Japan) and the Allies (France, Great Britain, the United States, the Soviet Union, and China).
Jun 8, 2021 · World War 2 involved the majority of the countries in the world, including all major powers, which formed two predominant alliances throughout the war: the Axis and the Allies. During the war, more than 30 countries with more than 100 million personnel served in uniform in various World War 2 countries.
World War II was the biggest and deadliest war in history, involving more than 30 countries. Learn about the causes, battles, leaders and impact of the war from 1939 to 1945.