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.