The Republic is a dramatic dialogue, not a treatise... of the city-soul analogy.
Socrates concludes that justice includes helping friends, but says the just man would never do harm to anybody (not even an enemy).
Socrates asks whether the ruler who makes a mistake by making a law that lessens the people's well-being, is still a ruler according to Thrasymachus' understanding of justice and injustice: "justice is what is advantageous to the stronger, while injustice is to one's own profit and advantage".
...Socrates' assertion that an artist (ruler) is someone who does his job well, and is a knower of some art, which allows him to complete the job well.
Accordingly, Socrates defines justice as "working at that which he is naturally best suited," and "to do one's own business and not to be a busybody" (433a-433b) and goes on to say that justice sustains and perfects the other three cardinal virtues: Temperance, Wisdom, and Courage, and that justice is the cause and condition of their existence. Socrates does not include justice as a virtue within the city, suggesting that justice does not exist within the human soul either, rather it is the result of a "well ordered" soul. A result of this conception of justice separates people into three types; that of the soldier, that of the producer, and that of a ruler. If a ruler can create just laws, and if the warriors can carry out the orders of the rulers, and if the producers can obey this authority, then a society will be just.
I think that the difficulty is in knowing exactly just what is a "just law".
I believe that humans will eventually learn how to control an enemy without harming the enemy - it will be an art - the art of survival.
The dialectical forms of government
While Plato spends much of the Republic having Socrates narrate a conversation about the city he founds with Glaucon and Adeimantus "in speech", the discussion eventually turns to considering four regimes that exist in reality and tend to degrade successively into each other: timocracy, oligarchy (also called plutocracy), democracy and tyranny (also called despotism).
Socrates defines a timocracy as a government of people who love rule and honor. Socrates argues that the timocracy emerges from aristocracy due to a civil war breaking out among the ruling class and the majority. Over time, many more births will occur to people who lack aristocratic, guardian qualities, slowly drawing the populace away from knowledge, music, poetry and "guardian education", toward money-making and the acquisition of possessions. This civil war between those who value wisdom and those who value material acquisition will be in struggle until a just medium is compromised. The timocracy values war insofar as it satisfies a love of victory and honor. The timocratic man loves physical training, and hunting, and values his abilities in warfare.
Temptations create a confusion between economic status and honor which is responsible for the emergence of oligarchy. In Book VIII, Socrates suggests that wealth will not help a pilot to navigate his ship, as his concerns will be directed centrally toward increasing his wealth by whatever means, rather than seeking out wisdom or honor. The injustice of economic disparity divides the rich and the poor, thus creating an environment for criminals and beggars to emerge. The rich are constantly plotting against the poor and vice versa. The oligarchic constitution is based on property assessment and wealth qualification. Unlike the timocracy, oligarchs are also unable to fight war, since they do not wish to arm the majority for fear of their rising up against them (even moreso fearing the majority than their enemies), nor do they seem to pay mercenaries, since they are reluctant to spend money.
As this socioeconomic divide grows, so do tensions between social classes. From the conflicts arising out of such tensions, the poor majority overthrow the wealthy minority, and democracy replaces the oligarchy preceding it. The poor overthrow the oligarchs and grant liberties and freedoms to citizens, creating a most variegated collection of peoples under a "supermarket" of constitutions. A visually appealing demagogue is soon lifted up to protect the interests of the lower class. However, with too much freedom, no requirements for anyone to rule, and having no interest in assessing the background of their rulers (other than honoring such people because they wish the majority well) the people become easily persuaded by such a demagogue's appeal to try and satisfy people's common, base, and unnecessary pleasures.
The excessive freedoms granted to the citizens of a democracy ultimately leads to a tyranny, the furthest regressed type of government. These freedoms divide the people into three socioeconomic classes: the dominating class, the elites and the commoners. Tensions between the dominating class and the elites cause the commoners to seek out protection of their democratic liberties. They invest all their power in their democratic demagogue, who, in turn, becomes corrupted by the power and becomes a tyrant with a small entourage of his supporters for protection and absolute control of his people.(Source.)