Stoicism is a form of attaining self authenticity through the rejection of own dreams and impossible plans, for the sake of acceptance of and reconciliation with painful conditions, whereby one exercises controls over only what he has power to control. Stoic is free when he is busy with controlling own passions while the world out there is beyond his control.
Sartre was distressed when he was mobilized on war in September 39. He was anxious with the perspective to be killed without having done anything great in his life. Under the impression produced by reading Heidegger he was all in the search of an "authentic position" in front of the face of danger. He first inclined towards becoming a stoicist (read "Notebooks from a Phoney War").
For Sartre that meant to quit his self-identification as a man meant to become great with his writings one day, and to put on sincerely the role of a soldier without love and future, meant to die.
While being in the army in 1939, and later in a camp for prisoners of war in 1940, he felt increasingly dissatisfied with the ostrich head-in-sand, "passive" (albeit of endurance) self-positioning. He began to strenghthen in his concept according to which everything we experience (including negative feelings) has been pre-processed and taken on by our (pre-reflective) consciousness, that everything can affect us not otherwise than only through us and via our "stamp". Therefore, if I feel e.g. helpless it is due to myself, my choice to be so. Thus, a man is responsible for everything which happens with him.
Sartre specially stipulates that the responsibility is not obvious to the first person perspective - not only because the choice is unreflective but also because it is not based on anything: consciousness is the basis of its possibilities and choices while consciousness itself exists by no ground; he labels this paradoxic yet fundamental human condition "unbearable" - since a man looks sentenced to be responsible for what happens with him in a way like he's just a receiver.
But as long as own reaction has been self-selected and the one is responsible for it the one ought to consciously charge himself with it as if it was voluntary chosen on purpose and as if for some progress to make about oneself. ("Notebooks from...", Mon., Dec. 4.)
There cannot be a world apriori/knowingly beyond control, because we can find ourselves only in such a world where our finger is in the pie already.
This existentialistic stance made Sartre a soldier/captive more active than he was initially under his stoicism. A stoicist will restrict himself with doing things he feels to be in control of; a Sartrian will expand himself doing things as if his circumstances are the condition of the whole world and are what he had brought in willingly, whatever is his control radius (the control and power will then get corrected via practical acts).