What does it mean to be a political realist in the eyes of philosophers such as Cicero, Aristotle, and Plato?

A political realist, in the eyes of Cicero, Aristotle, and Plato, would be someone who excessively discounts or rejects the idea that there are ethical and moral values on which political order can rest.
In the Melian Dialogue of Thucydides's History, the Athenians assert that the strong do what they can, and the weak suffer what they must. This is usually taken to be the first classic statement of political realism. Plato, by way of contrast, argued that a rational and just political order was possible but that it would require virtue.
Plato's student Aristotle also rejected realism, as his political science explicitly combined ethics with political philosophy, disciplines he considered inseparable. Cicero, too, wrote within this idealist tradition and spoke of natural law based on morality and the concept of just war in international relations.