During the joint German-Soviet invasion of Poland, which started World War II, on September 3 and 5, 1939, Zgierz was raided by Germany, and captured on September 6. Already in September 1939, the Germans committed first atrocities against Poles and carried out executions of Polish civilian defenders. Inhabitants of Zgierz were also among Poles murdered in nearby Łagiewniki on September 12 and in Retki on September 16. As part the "Intelligenzaktion", Germans carried out large massacres of Poles from the region in the nearby forests of Łagiewniki and Lućmierz, killing hundreds and thousands of people respectively. Germans also carried out expulsions of Poles and deported over 8,000 people to forced labour to Germany. Some were also killed in Nazi concentration camps, including the interwar director of the local State School of Economics, Jakub Stefan Cezak, and local Protestant parish priest, Aleksander Falzman. Schools were closed, factories were looted, Polish monuments were destroyed. Despite this, the Polish underground resistance movement was active in Zgierz.