For those who take any interest in history, philosophy, or politics, it is worth noting that by “State”, Plato and Aristotle meant city-state, i.e. a small and politically independent town. They did not mean the megalomanical nation-states that now dominate our planet.
For Plato, the ideal size of a “State” was around 5000 citizens/families. This is a number he picked not because he was provincial, but because it was a number large enough such that the city-state could efficiently sustain itself through division of labor, but small enough that it could have a common moral framework, and therefore harmony, amongst its citizens. Trade between city-states only increased prosperity, it was not necessary for survival and basic flourishing. A city-state must be neither too small nor too large, such that it is both a stable civilization and has absolute autonomy and freedom. It is the civilized counterpart to the roaming hunter, free to go where he chooses and eat what he can find, dependent only on himself and no other, subject to no rule but his own.
A city-state is the perfect entity for political experimentation, and therefore, of real progress in the realm of politics. A city-state is adaptable to the circumstance, its laws are based on the values of its citizens, if the laws become rotten the people can opt out by leaving, which encourages the improvement of those laws.
City-states are therefore, in principle, stable entities – so long as tyrannical nation-states don’t arise and prevent their natural, organic evolution. This last implies a reason to have strong federations of city-states – to defend themselves against such tyrannies. The other reason to have federations is to stop city-states experiments that have been carried too far, i.e. where they violate the natural rights of their own citizens (e.g. imprisoning dissenters, instituting slavery) or have violated the rights of the citizens of other city-states (e.g. engaging in piracy, looting, or trying to build an empire.)
The United States began as a very imperfect approximation to federations of city-states, but is now wildly careening down the path of empire.