This is a very complex matter, but I’ll try to give you a briefanswer. Ancient Greek Cities-States where a very small town or citywith its around territory part of them. Greeks would speak the samelanguage (in many dialects), have same religion, customs andbloodline but were very topicists and individual. Do not think big,like a modern Athens or Paris or London. Think that all Greece isalmost the same as the whole state of New York. Now think thatAthens is 1/3 of NYC. You get the picture. Imagine that Bronx,Kings, New York, Queens, Richmond were separate and individualcomplete independent countries as the Greek cities states of sameNY culture, but all waging war to each other and all togetheragainst Pennsylvania or Massachusetts. In order to achieve thisthey would form a Koinon (pron. keenon -> “commonwealth”) aconfederation or alliance for military reasons. e.g the Koinon ofNY so all would attack the city state of Boston or sometimes gobigger as the Koinon of North-East (like tenths of cities from NY,Pennsylvania, New Jersey, Massachusetts) and go to war against theKoinon of South East (like cities from Florida, Alabama, Georgia).Then the strongest or most influential culturally city wouldprevail and be in charge e.g the Delian League with Athensor Peloponnesian League with Sparta or the HellenicLeague with the Macedonians).
The Greeks, would 95% go to war against each other and not so oftenagainst other culture nations. The only notable time in antiquitythat they had a Greek Koinon as a whole, was with Alexander, whenthey conquered almost all known world.