Would Plato's city survive?

How can a city devoid of savagery survive through a barbarian invasion? In Plato's Greece, the fear was of such groups. Groups that had no care for philosophy or perfectly just republics, but only wanted to conquer as much as possible. Plato's proposed solution to the barbarian problem is a perfectly fit and trained warrior class. But the idea of death in Plato's society is not explained in the context of an inherent unfair nature of life, but in a natural occurrence that fits into the city's general love of order. There would be no order to savagery and the introduction of the apparent advantages of this disorder would implant themselves in the mind of the people. The people of the Republic are not taught why disorder is wrong, only that order is best. Once disorder is inevitably introduced to the people, it can only grow in influence.
I think one of Plato's major oversights is in his disregarding of the origin of vice. Vice, it seems to me, comes from a desire to avoid death and protect against risk, often overprotecting. Plato covers the protection from greed and even eliminates risk, but he does not account for his people's desire to avoid death even at the cost of others. If one in the city could realize that more food would better protect them as an individual from famine in times of drought, then that person would stockpile. Plato seems to think that famine would never reach his city. I know no place on Earth that is separate from these problems all the time. It only takes a little idea of perceived advantage to break the shield of Plato's defensive education on order. Plato's city would struggle to maintain order in face of savagery and death.

Plato's the Republic: Is Justice of Wisdom?

Plato has proved in the Republic that justice is not the interest of the stronger, the morality of the ruler, or the advantageous, but rather that justice is acting in one's own sphere and refraining from interfering in another's sphere. This definition works only in the context of Plato's republic wherein the idea of injustice, being the opposite of justice and not defined otherwise, has no place in the minds of the citizens. This kind of justice is not wisdom, but rather a kind of stagnation. Plato set out to mold a society made on the principles of justice only and not mutual advancement. The goal of Plato's society cannot be advancement because their is no understanding of gaining more than what is needed at any time. The principle of utilitarian placement of labor in society, a need of only subsistence, can only work if a society collectively buys in to that principle. Plato's society is not based on a want to destroy injustice in an already created one, but rather to create a society in which none of the citizens understand anything but their sphere.
Is this kind of society just in itself? Is it fair to deny the pursuit of knowledge and evolution to an entire populace? I don't believe it is and I don't believe it can work. I don't believe that injustice or justice are learned behavior. Humans have evolved through trial and error and the understanding of advantages. A society which contains merchants and exchange of goods and money affords opportunities for advantage to be found in these exchanges. The argument against this flaw is that the citizens of the republic will have no conception of unfairness or unfair dealings in exchange of goods. The will to be greater than others is an evolutionary maxim and cannot be taken out of a society.
And if it was, for argument's sake, is that just or good for the people of the republic? This principle denies any kind of evolution of man. The republic also does not exist isolated in the world. It will inevitably be a weak republic unless its citizens understand power. The warrior class of the republic would have to understand death and the search for a way in which to cheat death would open up avenues for injustice to creep in.
The denial of such supposed learned behaviors is not wise. It does not encourage a wise populace and it is unwise in itself. If a republic came to being such that it abided by the principle of subsistence as the only goal and everything remaining in its right place and still had an understanding of death as natural then Plato's republic could work. As it stands, Plato's just republic is unwise.