By 1939, when Bak was six years old, World War II began, and the city of Vilnius was transferred from Poland to Lithuania. When Vilnius was occupied by the Germans on June 24, 1941, Bak and his family were forced to move into the ghetto. At the age of nine, he held his first exhibition inside the ghetto. Bak and his mother sought refuge in a Benedictine convent where a Catholic nun named Maria Mikulska tried to help them. After returning to the ghetto, they were deported to a forced labour camp, but took shelter again in the convent where they remained in hiding until the end of the war.