On 1st September 1939 Poland was invaded by the Nazi Germany and World War II began. The Poles formed the strongest underground resistance movement in the occupied territories (Armia Krajowa (National Army)).
Many Polish soldiers joined military formations on the western and eastern fronts and fought together with the Allies against the German aggressor. In 1944 the Warsaw Uprising broke out being the largest single military effort taken by any European resistance movement of World War II. Six million of Polish citizens perished during the war, including three million of Polish Jews; the country lay in ruins.
Although the Soviet Union is usually named as one of the allied powers, that defeated Hitler-Germany, most Poles remember, that Soviet Union attacked Poland as an effect of the Molotov–Ribbentrop Pact on September 17th 1939. This unexpected attack once again divided Poland between eastern and western powers. Another atrocity from the Soviets was the Katyń massacre in 1940, where 22000 polish officers and other members of the intelligentsia was killed by the Soviet secret police. That the Soviet army was at Warsaws borders during the Warsaw Uprising and did not help the Poles is another reason, why the Poles see the Soviet Union not as a liberating rather as an occupying power.