[1][2][3][4] At the end of 1940, he organised a group of resistance fighters to write propaganda and published the underground newspaper Résistance {see image).
With Christian Pineau, he helped to create the Comité d’études économiques et syndicales (Economic and Trade Union Studies Committee) which became the Liberation-Nord movement.
He was appointed as a substitute deputy at the tribunal of the Seine in June 1941 and he combined this work with that of Versailles and with his activity as a resistance fighter.
During the summer of 1941, Liberation-Nord committed acts of sabotage, including sinking barges and blocking the Canal de Bourgogne at the Yonne river, a tributary of the Seine.
In March 1942, while he was in prison, his resistance group succeeded in lighting up the Renault factories in Boulogne-Billancourt at night so that they could be targeted and destroyed by a Royal Air Force bombardment.