During the joint German-Soviet invasion of Poland, which started World War II, Ostrów Mazowiecka was captured by Germany on September 8, 1939, and in mid-September the "Einsatzgruppe V" entered the town to commit various atrocities against the population. Already on 19 September the Germans arrested nine Poles, including chairmen of local veterans' organizations Włodzimierz Gadomski and Jan Radbalski. On 9 November 1939, one of the German soldiers set fire to the buildings on 3-ego Maja Street. The German authorities accused the Jews of starting the fire. On 11 November (though according to other sources, on 10 November) 500–800 Jews were killed. The Polish underground resistance movement was organized already in autumn of 1939. On May 25, 1943, the Polish resistance successfully assassinated the chief of the local German administration, in retaliation for which the Germans massacred about 140 Poles. The German occupation ended in 1944.