Beauvoir wrote Pyrrhus et Cinéas between the spring and autumn of 1943, alongside her novel The Blood of Others. Gallimard published it in September 1944, the first Beauvoir title released after the Liberation. It was her first commercial success — the small print run sold out within weeks — and it was widely reviewed as evidence that a distinct philosophical voice was emerging alongside Sartre's. Marybeth Timmermann's English translation appeared in the University of Illinois Press collection Philosophical Writings (Simone de Beauvoir Series, 2004), which is the standard English edition. There is no widely-available standalone English printing.
The essay opens on the anecdote it takes its title from — Cineas the adviser asking his conqueror-king Pyrrhus what he will do after Rome, after Carthage, after every named conquest, until Pyrrhus finally says he will rest. 'And why can't we rest now?' Cineas replies. The essay is Beauvoir's answer to Cineas: because rest, when it is achieved as the endpoint of a project, is not the same thing as the passive rest one already has before beginning. Human beings are the kind of being for whom the difference matters, and any ethics that does not begin from that fact will end in either the endless expansion of Pyrrhus (which cannot rest) or the passive retreat of Cineas (which cannot begin).
The essay is the first place Beauvoir works out the specific problem that would occupy the rest of her career: how does one act in a world already occupied by other free human beings, without either reducing them to tools of one's project or being reduced to the tool of theirs? The argument is not yet complete — she would spend The Ethics of Ambiguity three years later filling out the answer — but the shape of it is here. Every project needs other people; other people cannot legitimately be reduced to tools of that project; the affirmation of others' freedom is what makes one's own freedom worth having. Pyrrhus et Cinéas is a hundred-page compression of the philosophical work of the next thirty years.