During the German Invasion of Poland in 1939, the city was defended by a small garrison of four infantry battalions under General Konstanty Plisowski against General Heinz Guderian's XIX Panzer Corps. After four days of heavy fighting, the Polish forces withdrew southwards on September 17. The Soviet invasion of Poland began on the same day. As a result, the Soviet Red Army entered the city at the end of September 1939 following the Molotov–Ribbentrop Pact's Secret Protocol, and a joint Nazi-Soviet military parade took place on September 22, 1939. While Belarusians consider it a reunification of the Belarusian nation under one constituency (BSSR at that time), Poles consider it the date when the city was lost. During the Soviet control (1939–41), the Polish population was subject to arrests, executions and mass deportations to Siberia and the Kazakh Soviet Socialist Republic.