Stoicism began as a philosophy of private endurance and became the most politically consequential school of antiquity, largely by accident of what Rome did with it.
Zeno founded it in Athens around 300 BC, teaching in a painted colonnade that gave the school its name. Its physics held that the cosmos is ordered throughout by an immanent rational principle, the logos, and its ethics followed: to live well is to live in agreement with that order, which means distinguishing what is in our power from what is not and caring only about the former. Virtue is the sole good; everything else, including health, wealth and life itself, is indifferent.
Two political consequences follow, and they are the reason the school matters here. If reason is universal, then every human being possesses it, which makes the wise slave the equal of the foolish king and dissolves the natural standing of birth. And if the cosmos is governed by a law prior to any legislator, then human statutes are answerable to it. Cicero put both into Latin in De Legibus and De Re Publica: true law is right reason in agreement with nature, unalterable by senate or people, and the same in Rome and Athens.
That formulation passed into Roman jurisprudence as ius naturale, from there to Aquinas, and from Aquinas to Grotius, which is the pedigree of every later appeal to rights that hold whatever the law says. The Roman Stoa also produced its most-read texts, from a slave and an emperor: Epictetus and Marcus Aurelius. Its standing political weakness is quietism, since a doctrine of accepting what one cannot change makes an accommodating subject as easily as a principled one.