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World War II

World War II began with the Nazi German invasion of Poland on 1 September 1939, followed by the Soviet invasion of Poland on 17 September. On 28 September 1939, Warsaw fell. As agreed in the Molotov–Ribbentrop Pact, Poland was split into two zones, one occupied by Nazi Germany, the other by the Soviet Union. In 1939–1941, the Soviets deported hundreds of thousands of Poles. The Soviet NKVD executed thousands of Polish prisoners of war (among other incidents in the Katyn massacre) ahead of Operation Barbarossa. German planners had in November 1939 called for "the complete destruction of all Poles" and their fate as outlined in the genocidal Generalplan Ost.

Poland made the fourth-largest troop contribution in Europe, and its troops served both the Polish Government in Exile in the west and Soviet leadership in the east. Polish troops played an important role in the Normandy, Italian, North African Campaigns and Netherlands and are particularly remembered for the Battle of Britain and Battle of Monte Cassino. Polish intelligence operatives proved extremely valuable to the Allies, providing much of the intelligence from Europe and beyond, Polish code breakers were responsible for cracking the Enigma cipher and Polish scientists participating in the Manhattan Project were co-creators of the American atomic bomb. In the east, the Soviet-backed Polish 1st Army distinguished itself in the battles for Warsaw and Berlin.

The wartime resistance movement, and the Armia Krajowa (Home Army), fought against German occupation. It was one of the three largest resistance movements of the entire war, and encompassed a range of clandestine activities, which functioned as an underground state complete with degree-awarding universities and a court system. The resistance was loyal to the exiled government and generally resented the idea of a communist Poland; for this reason, in the summer of 1944 it initiated Operation Tempest, of which the Warsaw Uprising that began on 1 August 1944 is the best-known operation.

Nazi German forces under orders from Adolf Hitler set up six German extermination camps in occupied Poland, including Treblinka, Majdanek and Auschwitz. The Germans transported millions of Jews from across occupied Europe to be murdered in those camps. Altogether, 3 million Polish Jews – approximately 90% of Poland's pre-war Jewry – and between 1.8 and 2.8 million ethnic Poles were killed during the German occupation of Poland, including between 50,000 and 100,000 members of the Polish intelligentsia – academics, doctors, lawyers, nobility and priesthood. During the Warsaw Uprising alone, over 150,000 Polish civilians were killed, most were murdered by the Germans during the Wola and Ochota massacres. Around 150,000 Polish civilians were killed by Soviets between 1939 and 1941 during the Soviet Union's occupation of eastern Poland (Kresy), and another estimated 100,000 Poles were murdered by the Ukrainian Insurgent Army (UPA) between 1943 and 1944 in what became known as the Wołyń Massacres. Of all the countries in the war, Poland lost the highest percentage of its citizens: around 6 million perished – more than one-sixth of Poland's pre-war population – half of them Polish Jews. About 90% of deaths were non-military in nature.

In 1945, Poland's borders were shifted westwards. Over two million Polish inhabitants of Kresy were expelled along the Curzon Line by Stalin. The western border became the Oder-Neisse line. As a result, Poland's territory was reduced by 20%, or 77,500 square kilometres (29,900 sq mi). The shift forced the migration of millions of other people, most of whom were Poles, Germans, Ukrainians, and Jews.