During the joint German–Soviet invasion of Poland, which started World War II, in September 1939, Opatów was invaded and afterwards occupied by Germany. In 1939, the Germans ordered the demolition of the Ludwik Zwierzdowski monument, so the local mayor buried it to preserve it, thanks to which it was restored after the war. In 1940 the Germans carried out mass arrests of Poles as part of the "AB-Aktion". Poles arrested in March were either deported to Nazi concentration camps or massacred in Góry Wysokie, while those arrested in June were imprisoned and tortured in Skarżysko-Kamienna and then murdered. Opatów was a large center of the Polish underground resistance movement. Underground Polish press printed in nearby Sandomierz was distributed in Opatów since 1940, and from November 1943 a secret Polish printing house was also launched in Opatów. On March 11, 1943, the Germans carried out mass arrests of local Poles, including resistance members. On the night of March 12–13, 1943, a Polish underground partisan unit Jędrusie, together with soldiers of the Home Army, carried out an successful attack on the local German prison and liberated 80 prisoners, before the Germans would deport them elsewhere. In June 1944, the Home Army assassinated the local Gestapo chief Otto Schultz.