Opting not to return to France during the Russian Civil War, Sadoul co-founded the French Communist Group in Russia, fighting for control of it against Pierre Pascal and Henri Guilbeaux. Helping to set up the Red Army, he was sent to the Ukraine, where he instigated mutinies among the French intervention troops, and then to Germany, where he set up communist cells. Sadoul also mediated between the International and the SFIO's left-wing, attracting members for what became the French Communist Party (PCF), and contributed doctrinaire essays in "L'Humanité". A French military court sentenced him to death "in absentia", while the SFIO presented him, symbolically, as a candidate in the 1919 elections.