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On September 1, 1939, Hitler declared war by invading Poland, Britain and France, their allies, followed by declaring war on Germany.
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Working in concert with Hitler, Soviet leader Joseph Stalin orders the invasion of Poland, securing a share of Polish territory.
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Warsaw surrenders to German troops. Poland holds out for another 9 days before capitulating.
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The Soviet Union invades Finland, initiating the so-called Winter War. The Finns sue for an armistice and have to cede the northern shores of Lake Lagoda and the small Finnish coastline on the Arctic Sea to the Soviet Union.
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British cruisers defeat a German pocket battleship at the Battle of the River Plate, the first major naval engagement of World War II.
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Germany invades Norway, ending a 6-month period of limited land operations called the “Phony War.”
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Winston Churchill replaces Neville Chamberlain as British Prime Minister. The same day, Germany invades Belgium.
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France and the neutral Low Countries. Luxembourg is occupied on May 10; the Netherlands surrenders on May 14; Belgium surrenders on May 28. On June 22, France signs an armistice agreement by which the Germans occupy the northern half of the country and the entire Atlantic coastline. In southern France, a collaborationist regime with its capital in Vichy is established.
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The trapped British army evacuates to England from Dunkirk, France, surviving to fight another day.
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Paris falls to German forces. France capitulates 11 days later.
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The Soviet Union occupies the Baltic States on June 14–18, engineering Communist coup d’états in each of them on July 14–15, and then annexing them as Soviet Republics on August 3–6.
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The Soviet Union forces Romania to cede the eastern province of Bessarabia and the northern half of Bukovina to Soviet Ukraine.
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The Battle of Britain pits German bombers against British fighters in a thwarted German prelude to invasion.
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Japan attacks Pearl Harbor, starting war with the US. Sensing weakness, Hitler declares war on America 4 days later.