Realism is a school of international relations that holds that states always act out of a desire to increase their security and their power. This is a school that does not concern itself with issues of the morality of state action. In essence, states should do whatever is necessary to increase their power and security because they exist in an anarchic order where there is no authority to protect them from the aggression of others. In the Melian Dialogue (part of Thucydides’ History of the Peloponnesian War), the Athenians say things that reflect realist ideas.
In general, the Athenians are saying that they have enough power to be able to tell the Melians what to do. They are saying that the Melians should give in because they are weak. They are saying that issues of morality are not important in this case.
For example, when the Melians ask about the equity of a certain course of action, the Athenians say
… if any maintain their independence it is because they are strong, and that if we do not molest them it is because we are afraid…
The Athenians also say that people will always “rule wherever they can” and that this is why the Athenians know that the Melians
and everybody else, having the same power as we have, would do the same as we do…
In these ways, this dialogue reflects the idea that states do what they are able to do to maintain power and security, regardless of issues of morality.