While the broad focus of Sartre's life revolved around the notion of human freedom, he began a sustained intellectual participation in more public matters towards the end of the Second World War, around 1944–1945. Before World War II, he was content with the role of an apolitical liberal intellectual: "Now teaching at a lycée in Laon ... Sartre made his headquarters the Dome café at the crossing of Montparnasse and Raspail boulevards. He attended plays, read novels, and dined [with] women. He wrote. And he was published." Sartre and his lifelong companion, de Beauvoir, existed, in her words, where "the world about us was a mere backdrop against which our private lives were played out".
Sartre portrayed his own pre-war situation in the character Mathieu, chief protagonist in The Age of Reason, which was completed during Sartre's first year as a soldier in the Second World War. By forging Mathieu as an absolute rationalist, analyzing every situation, and functioning entirely on reason, he removed any strands of authentic content from his character and as a result, Mathieu could "recognize no allegiance except to [him]self", though he realized that without "responsibility for my own existence, it would seem utterly absurd to go on existing". Mathieu's commitment was only to himself, never to the outside world. Mathieu was restrained from action each time because he had no reasons for acting. Sartre then, for these reasons, was not compelled to participate in the Spanish Civil War, and it took the invasion of his own country to motivate him into action and to provide a crystallization of these ideas. It was the war that gave him a purpose beyond himself, and the atrocities of the war can be seen as the turning point in his public stance.
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".
The symbolic initiation of this new phase in Sartre's work is packaged in the introduction he wrote for a new journal, Les Temps modernes, in October 1945. Here he aligned the journal, and thus himself, with the Left and called for writers to express their political commitment. Yet, this alignment was indefinite, directed more to the concept of the Left than a specific party of the Left.
Sartre's philosophy lent itself to his being a public intellectual. He envisaged culture as a very fluid concept; neither pre-determined, nor definitely finished; instead, in true existential fashion, "culture was always conceived as a process of continual invention and re-invention." This marks Sartre, the intellectual, as a pragmatist, willing to move and shift stance along with events. He did not dogmatically follow a cause other than the belief in human freedom, preferring to retain a pacifist's objectivity. It is this overarching theme of freedom that means his work "subverts the bases for distinctions among the disciplines". Therefore, he was able to hold knowledge across a vast array of subjects: "the international world order, the political and economic organisation of contemporary society, especially France, the institutional and legal frameworks that regulate the lives of ordinary citizens, the educational system, the media networks that control and disseminate information. Sartre systematically refused to keep quiet about what he saw as inequalities and injustices in the world."
Sartre always sympathized with the Left, and supported the French Communist Party (PCF) until the 1956 Soviet invasion of Hungary. Following the Liberation the PCF were infuriated by Sartre's philosophy, which appeared to lure young French men and women away from the ideology of communism and into Sartre's own existentialism. From 1956 onwards Sartre rejected the claims of the PCF to represent the French working classes, objecting to its "authoritarian tendencies". In the late 1960s Sartre supported the Maoists, a movement that rejected the authority of established communist parties. However, despite aligning with the Maoists, Sartre said after the May events: "If one rereads all my books, one will realize that I have not changed profoundly, and that I have always remained an anarchist." He would later explicitly allow himself to be called an anarchist.
In the aftermath of a war that had for the first time properly engaged Sartre in political matters, he set forth a body of work which "reflected on virtually every important theme of his early thought and began to explore alternative solutions to the problems posed there". The greatest difficulties that he and all public intellectuals of the time faced were the increasing technological aspects of the world that were outdating the printed word as a form of expression. In Sartre's opinion, the "traditional bourgeois literary forms remain innately superior", but there is "a recognition that the new technological 'mass media' forms must be embraced" if Sartre's ethical and political goals as an authentic, committed intellectual are to be achieved: the demystification of bourgeois political practices and the raising of the consciousness, both political and cultural, of the working class.
The struggle for Sartre was against the monopolising moguls who were beginning to take over the media and destroy the role of the intellectual. His attempts to reach a public were mediated by these powers, and it was often these powers he had to campaign against. He was skilled enough, however, to circumvent some of these issues by his interactive approach to the various forms of media, advertising his radio interviews in a newspaper column for example, and vice versa.
The role of a public intellectual can lead to the individual placing himself in danger as he engages with disputed topics. In Sartre's case, this was witnessed in June 1961, when a plastic bomb exploded in the entrance of his apartment building. His public support of Algerian self-determination at the time had led Sartre to become a target of the campaign of terror that mounted as the colonists' position deteriorated. A similar occurrence took place the next year and he had begun to receive threatening letters from Oran, Algeria.