He is popularly known as one of the founders of Existentialism, but even his one-time friend Camus was not happy with the label. Sartre himself recognised its limitations when he was captured by the Germans and his freedom curtailed as a prisoner of war. But it also meant that he met a wider cross-section of people than the privileged intellectual elite he'd been part of and valued the feeling of solidarity. This led to his interest in marxism but also his difficult relations with the the French Communist Party.

From a review of Ian Birchall's "Sartre Against Stalinism", by Rebecca Pitt

"Sartre's earlier works such as Existentialism and Humanism and Being and Nothingness express 'Sartre's basic message--that the world can be changed; that we are free to change it; and if we fail to do so we bear the responsibility'...

... the head of the PCF, Maurice Thorez, for example, had condemned Sartre's ideas as 'the existentialist putrescence'--showing the threat that the PCF perceived from Sartre.17

That Sartre's interest in freedom was not purely philosophical is made clear in the example of Sartre's 1943 occupation play The Flies. The Flies originates from the Greek myth of Electra and Orestes, and discusses the nature of fate and freedom via a comparison of Electra and Orestes' reactions to matricide committed as revenge. Yet the play, Sartre commented in 1947, was an attempt to show that 'remorse was not the attitude Frenchmen should choose after our country's military collapse. Our past no longer existed... But the future--even though an enemy army was occupying France--was new...we were at liberty to make it a future of the defeated or a future of free men'.18 Birchall recognises there was a tactical reason why Sartre chose to initiate performances of The Flies during the occupation. The Flies was intended to make clear to the French people that they were responsible for how they responded to the occupation. With this in mind, it becomes obvious that the writer Gilbert Joseph's criticism of Sartre for collaborating with occupying forces by allowing his work to be performed fails to understand the very objective of the play.19

Putting Sartre in perspective

In 1974 Simone de Beauvoir (philosopher, novelist and author of The Second Sex) asked Sartre how he perceived the relation between himself and organised politics. He replied:

Whenever I committed myself in one way or another to politics and carried out an action, I never abandoned the idea of freedom. On the contrary, every time I acted I felt free. I've never belonged to a party... I have...been in touch with various groups but without belonging to them.20

This makes it clear that there was never a formalised relationship between Sartre and the PCF following their 'four-year romance' in the 1950s. He refers in the same interview to French Maoism--which he was interested in during the 1968 period--and to the fact that he could not completely 'commit' himself to one political organisation. In the introduction to his book, Birchall discusses why this may have been the case, pointing to the nature of revolutionary organisations as being 'always the choice of the future against the present' as a possible cause."

http://pubs.socialistreviewindex.org.uk/isj102/pitt.htm

Maybe it's because I'm a Londoner - that I moved to Nice.