The polis (city state)
Classical Greece was divided into polis (city-states) with each polis having its own form of government. Some city-states, like Corinth, were ruled by kings. Some, like Sparta, were ruled by a small group of men. Others, like Athens, experimented with new forms of government. This arrangement was viewed with approval by prestigious philosophers like Plato and Aristotle who believed a man identified himself first and foremost as a citizen of a particular polis and that his destiny was tied to his fellow inhabitants and the common good of the city.
Citizens of the world
Stoics emphasized the cosmos which they viewed as a single living entity of which we are all a part and united to each other by our shared humanity. Hence Stoic philosophizers were fond of proclaiming 'the cosmos is my polis', echoing Diogenes the Cynic when he proclaimed "I am a citizen of the world."
But for Diogenes being a citizen of the world meant rejecting duties and responsibilities traditionally expected of citizens in a classical Greek city-state. Stoics disagreed with Diogenes and to fully understand why we must take a slight detour into another key concept of their philosophy : oikeiosis.
Oikeiosis has no literal translation into English. The Encyclopedia Britannica translates it as "love of self"; the Stanford Encyclopedia prefers the term "natural attachment to what is appropriate" and notes the "Greek term ‘oikeion’ can mean not only what is suitable, but also what is akin to oneself, standing in a natural relation of affection."
As children we are naturally drawn towards food and warmth. When we mature we are drawn towards human companionship because humans are a social animal and we discover other pursuits appropriate to our nature, as both individuals and as humans. Thus one human may be drawn to writing or philosophy or to sport or business. This is in contrast to the modern fascination with finding yourself: for the stoic, life will teach you what you need to know.
We are now in a position to understand cosmopolitanism from a stoic viewpoint.
If mind is common to us, then also the reason, whereby we are reasoning beings, is common.' If this be so, then also the reason which enjoins what is to be done or left undone is common. If this be so, law also is common; if this be so, we are citizens; if this be so, we are partakers in one constitution; if this be so, the Universe is a kind of Commonwealth. - Marcus AureliusAs social creatures we are naturally drawn to other humans regardless of their location. As rational creatures we understand we are all part of the cosmos, united together by virtue of our common humanity. As citizens of the cosmos we have duties and responsibilities to each other for the common good, thus we must serve each other. The Stoics believed the best path to serving each other was either through political engagement or as a tutor teaching virtue to others. It is easy to see why stoic cosmopolitanism flourished throughout the Roman Empire that united huge territory under a single political system and the relevance of Stoic cosmopolitanism to modern institutions like the united nations and the European parliament.