Complex sentence: He narrowly escaped execution only days before The Children's Action, of March 27-28,1944 when Nazi troops rounded-up all children in the Kovno camp and marched them to the Ninth Fort where they were shot or sent to Auschwitz to be gassed.
Split into simpler sentences: He narrowly escaped execution only days before The Children's Action, of March 27-28,1944. During this Action Nazi troops rounded-up all children, marched them to the Ninth Fort and shot them or sent them to Auschwitz to be gassed.