On the day of the outbreak of World War II in September 1939, Chorzów was taken by Nazi Germany. Polish irregulars, mainly Silesian uprising veterans and scouts, put up resistance to the regular German forces for three days, afterwards the city was occupied by Germany, and on September 6, 1939, the "Einsatzgruppe I" entered the city to commit various atrocities against Poles. Most of the Polish defenders were murdered in mass executions. An execution of three Poles was carried out by the German "Freikorps" already on September 3, 1939. A unit of the "Einsatzgruppe I" was stationed in Chorzów, and it was responsible for many crimes against Poles committed in Chorzów and the nearby cities of Czeladź and Siemianowice Śląskie. Polish property was confiscated, and Chorzów was promptly re-incorporated into German Silesia; the Upper Silesian industry being one of the pillars of the Nazi Germany war effort. In 1939 and 1940, the Germans carried out mass arrests of Polish intelligentsia, especially teachers, for which a prison was operated in the city (see "Intelligenzaktion"). Local Polish teachers were among Poles murdered in 1939 in Chorzów and Strzybnica (present-day district of Tarnowskie Góry), and later in the Dachau concentration camp.