In 1940 Baum was rounded up and forced into slave labour at the electromotive works of the Siemens-Schuckertwerke (today Siemens AG).

From 1941, he headed a group of Jewish slave labourers at the plant, who, to escape deportation to concentration camps, went into the Berlin underground.

On 18 May 1942, the group organised an arson attack on an anticommunist and anti-Semitic propaganda exhibition known as The Soviet Paradise prepared by Joseph Goebbels at the Berliner Lustgarten.

The attack was only partially successful and, within days, a large number of the group's members were arrested and 20 were sentenced to death.

[3] Listing of some of the Baum Group's members with their dates of birth and death; they were all executed in Berlin-Plötzensee on March 4, 1943: Heinz Rotholz (1922–1943), Heinz Birnbaum (1920–1943), Hella Hirsch (1921–1943), Hanni Meyer (1921–1943), Marianne Joachim (1922–1943), Lothar Salinger (1920–1943), Helmut Neumann (1922–1943), Hildegard Löwy and Siegbert Rotholz (1922–1943).