After coming back to Paris in May 1941, he participated in the founding of the underground group Socialisme et Liberté ("Socialism and Liberty") with other writers Simone de Beauvoir, Maurice Merleau-Ponty, Jean-Toussaint Desanti, Dominique Desanti, Jean Kanapa, and École Normale students. In spring of 1941, Sartre suggested with "cheerful ferocity" at a meeting that the Socialisme et Liberté assassinate prominent war collaborators like Marcel Déat, but de Beauvoir noted his idea was rejected as "none of us felt qualified to make bombs or hurl grenades". The British historian Ian Ousby observed that the French always had far more hatred for collaborators than they did for the Germans, noting it was French people like Déat that Sartre wanted to assassinate rather than the military governor of France, General Otto von Stülpnagel, and the popular slogan always was "Death to Laval!" rather than "Death to Hitler!". In August Sartre and de Beauvoir went to the French Riviera seeking the support of André Gide and André Malraux. However, both Gide and Malraux were undecided, and this may have been the cause of Sartre's disappointment and discouragement. Socialisme et liberté soon dissolved and Sartre decided to write instead of being involved in active resistance. He then wrote Being and Nothingness, The Flies, and No Exit, none of which were censored by the Germans, and also contributed to both legal and illegal literary magazines.
The war opened Sartre's eyes to a political reality he had not yet understood until forced into continual engagement with it: "the world itself destroyed Sartre's illusions about isolated self-determining individuals and made clear his own personal stake in the events of the time." Returning to Paris in 1941 he formed the "Socialisme et Liberté" resistance group. In 1943, after the group disbanded, Sartre joined a writers' Resistance group, in which he remained an active participant until the end of the war. He continued to write ferociously, and it was due to this "crucial experience of war and captivity that Sartre began to try to build up a positive moral system and to express it through literature".