Upon his release, he immediately went to Hamburg, where he found work at a shipyard and got back in touch with friends
Bernhard Bästlein and
Robert Abshagen, with whom he formed the
Bästlein-Jacob-Abshagen Group, a Communist
resistance group. He again was responsible for agitation and propaganda, producing flyers and other publications. He also began creating an archive for the group, which he conspired with a friend, Otto Gröllmann, who was a set designer at the
Thalia Theater (Hamburg) to conceal there. The archive has since been lost. After a wave of arrests began in Hamburg in October 1942, which included Bästlein and Abshagen, Jacob fled and went to Berlin, where he was again underground. In 1943, he formed another resistance group, this time with
Anton Saefkow. Bästlein was able to escape prison during a bombing raid in 1944 and ran into Jacob by chance, after which he joined them in forming the
Saefkow-Jacob-Bästlein Organization, They focused on disseminating information that they were able to glean from foreign newspapers and from radio broadcasts from Moscow. They also organized the
Bewegung Freies Deutschland (Free Germany Movement) to work with people in factories, military units, opposition parties and others, growing to several hundred people. In his publication,
Am Beginn der letzten Phase des Krieges ("At the beginning of the last phase of the war"), Jacob wrote that to end the war and overthrow the fascist dictator, Communists should concentrate all their strength "on developing a broad, national front composed of all groups that stand opposed to fascism. Jacob lived underground in Berlin almost two years, moving frequently, some 30 times in 18 months, and having to remain very quiet during the daytime, so as not to be overheard. Being illegal, meant he also had to sit out bomb raids and dared not seek cover in a bomb shelter. == Plot to kill Hitler and Jacob's death ==