Plato wrote when Athens was defeated, democracy was in disrepute (to some). But the Greeks had also shown that they were capable of defeating a huge empire, the Persians. Twice. To follow up on that, there would have to be changes to the loose collection of states that was Greece.
“...εἴ τις λάβοι παρὰ φίλου ἀνδρὸς σωφρονοῦντος ὅπλα, εἰ μανεὶς ἀπαιτοῖ, ὅτι οὔτε χρὴ τὰ τοιαῦτα ἀποδιδόναι, οὔτε δίκαιος ἂν εἴη ὁ ἀποδιδούς, οὐδ᾽ αὖ πρὸς τὸν οὕτως ἔχοντα πάντα ἐθέλων τἀληθῆ λέγειν.”
“...if one took over weapons from a friend when he was sane and when mad, he demanded them back: then we ought not to return them; he who did return them would not be acting justly, nor would he choose to tell the whole truth to such a person.”
This was in answer to Cephalus' statement that justice is "paying your debts". The context is that Athens had built itself a large fleet and the crew of the ships was mainly made up of lower-class citizens, those who would in the past not have been eligible to be in the army, not having the wealth to buy themselves a suit of armour. For many of the elite, this was one step too far. Participation in the army also brought political power.
The defining characteristic of the city-state model that was born roundabout Homer's time: the polis does not have an army, it is an army. Not a collection of private armies but a unity, consisting of all able men who can afford a set of armour. The model is flexible enough to vary from place to place but its basic features are the same everywhere. A man's honour is dependent on being a part of this and he, even if he is Achilles, has to obey the appointed commander no matter how arrogant or incompetent he thinks this commander is. Honour and shame are the two complementary forces that make this system possible and they come from the fellow citizens(0). This is the city-state model, it goes with the Olympian religion and it maintains an open society(1).
The difference is worked out by Plato in his "Republic". Here he creates two cities, first a simple one with no strong hierarchy or specialization of tasks, no special ambitions and only a few luxuries. Socrates calls it "the true city" but his conversation partners disparagingly call it "city of sows": they want more than that. So the rest of the "Republic" discusses what it would take to maintain a better, bigger city.
Leo Strauss has an interesting comment to make here: he says(1) that the first city "lacks virtue": no hymns to the Good, "niceness is not a virtue", Justice as fairness in buying and selling is only a lower kind, a "money-earner's virtue". And he explains himself later: Virtue requires the presence of Evil. I think here he has put his finger on the central issue of it all: the Polis, the Homeric city-state, does not know Evil or Virtue (see my Introduction). The "niceness is not a virtue" statement is a bit of a misser: the historical Greek city-states were not particularly nice. The Polis, and therefore Plato's first city, were armies, seated but still fully armed and they could be harsh with the best of them. But they, taught by Homer, had not split the world into Good and Evil and that makes a lot of difference. That split remained to be made by Plato.
The "kingdom" model is very different: a king has an army like a shepherd has dogs. The purpose of his dogs is to defend his sheep against predators but also, importantly, to control the sheep. Well trained, fierce but obedient and sheep-friendly dogs are essential for maintaining a good herd. All this is worked out in Plato's scary masterpiece. We would be very wrong however if we took his "republic" as a straightforward model of Utopia. First of all: our translation of the title is wrong. It is not about a republic with Socrates as president, it discusses the model of a kingdom and the problems of that model. It would go much too far to discuss all this book has to say (or even to claim that I understood all it has to say) but the basics are clear enough. Plato wants to close society again. This means that "inside" and "outside" have to be clearly distinguished. Inside is Good, outside is Evil. But Evil cannot always be kept outside, so he wants to give the weapons (the army) back to the ones who have been trained to know Good and Evil. The city will be the army no longer - which is indeed what happens not very long after his death. The citation at the top of the page must represent Plato's life experience and the "Republic" his discussion of what it would take to avoid the errors of the Thirty. For this purpose, not only the city but the very gods have to be purified so the Good can rule. A new city where, and this is the unstated but unavoidable conclusion, not only the poet but also the philosopher will be unwelcome...
...except as king of course. This is partly his boast of what philosophy can do, partly his ironical way of warning us that the king of this polis will have to be like a certain philosopher in his unselfish devotion to the common good. But any absolute ruler, once he rules, is by definition a (or rather the) philosopher: he knows what is good and just for us. A Socrates who questions any of this is even more unwelcome than a big mouth poet.
“τὸ τὰ αὑτοῦ πράττειν” - irreverently translated as "mind your own business" - is the foundation of Plato's Kallipolis. Only the King can mind everybody's business. One might object that this is "good" rather than "just". But for a clear view, let me quote Leo Strauss:
The artisan in the strict sense is infallible, i.e. does his job well, and he is only concerned with the well-being of others. This however means that art strictly understood is justice - justice in deed and not merely justice in intention as law-abidingness is. “Art is justice” - this proposition reflects the Socratic assertion that virtue is knowledge. The suggestion emerging from Socrates’ discussion with Thrasymachus leads to the conclusion that the just city will be an association in which everyone is an artisan in the strict sense, a city of craftsmen or artificers, of men (and women) each of whom has a single job which he does well and with full dedication, i.e. without minding his own advantage, only for the good of others or for the common good. This conclusion pervades the whole teaching of the Republic. The city constructed therein as a model is based on the principle “one man one job” or “each should mind his own business.”“”
from Leo Strauss: The City and Man - on Plato's Republic pg 79
We should not underestimate
the force of this model. Every larger, hierarchical organization (e.g. a major company, a
government department) in the modern world builds
on it. Everybody does their job and does not interfere, neither upward in the hierarchy nor
left and right. Downward interference, of course, is our business. Inside the organization
there should be no question what is good: it is the good of the whole. NB: not society
or the world as a whole, only the "cosmos" which is this organization. Also: "what happens here,
The good, the true, the beautiful and most of all justice are defined only within the
boundaries of the system. It is essential that these "gods" are obeyed for the organization
will perish if they are not. The above-mentioned organizations obey the higher law,
the law of the state or of God, only by compulsion from outside.
They do not naturally do so.
Clearly, this is the justice of the gang of bandits (Rep. 351c). Law inside, free-for-all outside. Plato knows that there is more to it than that (for instance, the analogy soul - city breaks down here), so he invents a reward-and-punishment-after-death kind of justice. "You cannot have it here but, believe me, the crooks will be punished in the hereafter".
This model is also the foundation of our nation-state. The justice inside we manage to maintain reasonably well so far, our foreign policy is still based on self-interest only: the world is a community of gangs of bandits. The ancient Greek attempt to make the city-states live together under the reign of Zeus, failed. We will have to try again, or wait until some power conquers us all.