One of the concepts in Stoicism that receives less attention than perhaps it ought in modern times is the concept of the "brotherhood of man" - the essential obligation of all human beings to all other human beings, simply by virtue of being human. Yes, human beings, lacking perfect wisdom, will quarrel and even go to war with one another, yet we are all human, and all have a duty to treat one another as fellow human beings. Here is Margaret R. Graver's take on this from Stoicism and Emotion:
"[E]very person has an obligation to consider the interests of others in determining how to act. In theory, that obligation should extend not only to others in existing social systems (the polis within which one is born) but also to every human being, since all rational beings are in fact united in a single cosmic community under the rule of Zeus. Progress in ethical understanding is in large part a matter of increasing one's awareness of the extent of this obligation. "Every human being should regard every other as akin just because they are human," writes Cicero." Another fragment by Hierocles speaks of a sense of kinship (again, oikeiosis) as something that can be intensified by conscious effort. Hierocles thinks of the individual as surrounded by concentric circles representing successively the self, the nuclear family, the extended family, the neighborhood, city, country, and finally the whole human race. It is a mark of character, he says, "to somehow pull the circles toward the center in one's proper treatment of each person, deliberately transferring those in the outer circles to the inner ones. As one comes to think of the persons in the wider circle as truly belonging to oneself, one will be increasingly motivated to behave toward them in the way the wise person would do."
Margaret R. Graver. Stoicism and Emotion (pp. 176-177). Kindle Edition.