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The Soviet invasion of Poland was a military operation by the Soviet Union without a formal declaration of war. On 17 September 1939, the Soviet Union invaded Poland from the east, sixteen days after Germany invaded Poland from the west. Subsequent military operations lasted for the following 20 days and ended on 6 October 1939 with the two-way division and annexation of the entire territory of the Second Polish Republic by Nazi Germany and the Soviet Union. This division is sometimes called the Fourth Partition of Poland. The Soviet (as well as German) invasion of Poland was indirectly indicated in the "secret protocol" of the Molotov–Ribbentrop Pact signed on 23 August 1939, which divided Poland into "spheres of influence" of the two powers and questioned the future existence of the Polish state. German and Soviet cooperation in the invasion of Poland has been described as co-belligerence.
The Red Army, which vastly outnumbered the Polish defenders, achieved its targets encountering only limited resistance. Some 320,000 Polish prisoners of war had been captured. The campaign of mass persecution in the newly acquired areas began immediately. In November 1939 the Soviet government annexed the entire Polish territory under its control. Some 13.5 million Polish citizens who fell under the military occupation were made into new Soviet subjects following show elections conducted by the NKVD secret police in the atmosphere of terror, the results of which were used to legitimize the use of force. A Soviet campaign of political murders and other forms of repression, targeting Polish figures of authority such as military officers, police and priests, began with a wave of arrests and summary executions. The Soviet NKVD sent hundreds of thousands of people from eastern Poland to Siberia and other remote parts of the Soviet Union in four major waves of deportation between 1939 and 1941. Soviet forces occupied eastern Poland until the summer of 1941, when they were driven out by the German army in the course of Operation Barbarossa. The area was under German occupation until the Red Army reconquered it in the summer of 1944. An agreement at the Yalta Conference permitted the Soviet Union to annex almost all of their Molotov–Ribbentrop Pact portion of the Second Polish Republic, compensating the Polish People's Republic with the greater southern part of East Prussia and territories east of the Oder–Neisse line. The Soviet Union appended the annexed territories to the Ukrainian Soviet Socialist Republic, the Byelorussian Soviet Socialist Republic and the Lithuanian Soviet Socialist Republic.
After the end of World War II in Europe, the Soviet Union signed a Polish–Soviet border agreement with the new, internationally recognized Polish Provisional Government of National Unity on 16 August 1945. This agreement recognized the status quo as the new official border between the two countries with the exception of the region around Białystok and a minor part of Galicia east of the San River around Przemyśl, which were later returned to Poland.