During the Second World War and the 1941 Nazi occupation of Yugoslavia, Slavko's father was arrested by the Ustaša, while Slavko escaped from Karlovac to where he was hidden by the Djerek family. His brother Danko took his grandfather Aron to Tuzla, while his mother Lea was in jail from July to August, when she was released thanks to some friends. In January 1942, all three were found in Kraljevica. They thus joined the Partisans with whom they stayed until the end of the war. From Spring 1942 to 1945, Goldstein was active in the field and combat units of the NOVJ. He ended the war, at 17, with the rank of Lieutenant.