In World War II, during the Warsaw Uprising, like nearby St. Hyacinth's Church, St. Kazimierz was used as a hospital. The nuns housed many civilians in the church and cellars and gave medical aid as well as they could. In August 1944 they decided to suspend their rules to assist wounded insurgents. Because of this decision the German military targeted the church and the cloister for heavy bombing. During a single raid on August 31, 1944, four priests, 35 nuns and over a thousand civilians sheltering in the church's crypt were killed. Sobieska's magnificent church and cloisters was destroyed.