Soviet invasion of Poland

The Soviet invasion of Poland was the attack by the Soviet Red Army on the eastern territory of the Second Polish Republic, launched on 17 September 1939 without a formal declaration of war, sixteen days after Germany invaded from the west. The campaign lasted about twenty days and ended on 6 October 1939 with the division and annexation of Polish territory between Germany and the Soviet Union, an outcome sometimes called the Fourth Partition of Poland. Its background was the secret protocol of the Molotov-Ribbentrop Pact of 23 August 1939, which divided Poland into spheres of influence. The Red Army greatly outnumbered the Polish defenders, met limited resistance, and took about 320,000 Polish prisoners of war. The Soviet Union annexed the occupied territory in November 1939, and mass arrests, executions and deportations followed. The area stayed under Soviet control until Germany invaded the Soviet Union in 1941.