Niccolò Machiavelli

Simply being good in order to do good is not enough. That is true. But simply being skilled—to have what Machiavelli calls virtú (as opposed to virtue)—in order to do good is also not enough.

Realism and the just war ethic both pursue war in certain contexts, so they can appear to be close cousins. But the just war ethic shares less with realism than realism shares with pacifism.