Jewish Combat Organization
In 1939, after the German invasion of Poland Edelman found himself confined—along with the other Jews of Warsaw—to the Warsaw Ghetto. In 1942, as a Bund youth leader he co-founded the underground Jewish Combat Organization ("Żydowska Organizacja Bojowa", ŻOB). In the Warsaw Ghetto Uprising of April–May 1943, led by Mordechaj Anielewicz, Edelman was one of the three sub-commanders and then became the leader after the death of Anielewicz.