For Machiavelli, 'virtu' or skill as wielded by a ruler would make for a state that would thrive and protect itself from the machinations of enemy-states. War for Plato is essentially unjust and the only justification for it is for defending the state, while for Machiavelli a state exists to wage war against other states - thus, war, is a natural condition in a state's existence. Plato and Machiavelli also lived in different eras. For the former, philosophy is concerned with the truth, as embodied by principles and how things should be, and this concern would naturally result in a perfect society. For the latter, philosophy is allied with the realities of power, in how things are as they are.
Philosophers in Plato's Republic are the only ones ideal to rule (and become kings) because they love and search for the truth - as opposed to the other two classes, the people who are mainly concerned with honor, and the masses, who are concerned with money and the indulgence of physical appetites. The philosopher-kings possess the quality of truthfulness who "will never intentionally receive into their minds falsehood, which is their detestation, and they will love the truth." Truth as conceived by Plato is absolute, dealing with the eternal and the unchanging, the "forms" opposed to the fickle, the merely seen and experienced.