Following the Nazi German and Soviet invasion of Poland, which started World War II, Kolno was taken over by the German forces on 8 September 1939. On 12–13 September 1939, the German "Einsatzgruppe V" entered the town to commit various crimes against the populace. On 29 September Soviets entered the area in accordance with the Ribbentrop-Molotov Pact. The Soviets carried out mass deportations of the inhabitants to the Soviet Union. Already in 1939, the Polish resistance movement was organized in Kolno by Stanisław Milewski. The town remained under Soviet occupation until Operation Barbarossa (22 June 1941) when it was overrun again by the Wehrmacht.