An inherent part of the Sovietization was a rule of terror started by the NKVD and other Soviet agencies . The first victims of the new order were approximately 250,000 Polish prisoners of war captured by the USSR during and after the Polish Defensive War ( see Polish prisoners of war in Soviet Union ( after 1939 ) ) . As the Soviet Union did not sign any international convention on rules of war , they were denied the status of prisoners of war and instead almost all of the captured officers and a large number of ordinary soldiers were then murdered ( see Katyn massacre ) or sent to Gulag . Thousands of others would fall victim to NKVD massacres of prisoners in mid-1941 , after Germany invaded the Soviet Union
Katyn massacre
Katyn massacre
Occupation of Poland (1939–1945)