Should nations act in their own interests or in pursuit of idealistic goals? This is the fundamental issue dividing international relations. This is not a Left versus Right or liberal versus conservative issue, though these things are involved.
There are conservative liberal internationalists and there are realist leftists.
What do these terms mean and how can we better understand the goals behind each group.
Realists believe countries should and do act in their own interests, even when they claim otherwise. They are skeptical about utopian dreams of peace through trade and democracy. Realist hold Machiavelli as their founder and reference the Melian Debate from Thucydides’ History of the Peloponnesian War.
Athens had organized a “defensive league” to fight against Persia, which had invaded Greece multiple times and controlled the Greek cities of Asia Minor. This defensive league had become the Athenian Empire and Sparta and others allied to resist Athenian hegemony. Athens was a sea power and a trade power. Athens also claimed to be a democracy and saw Sparta as a monarchy.
A small island called Melos was a Spartan colony and just wanted to be neutral in the conflict. Athens can with a large fleet and army to make Melos join its league. The debate is recorded in the book and one section is key:
since you know as well as we do that right, as the world goes, is only in question between equals in power, while the strong do what they can and the weak suffer what they must.
Melos refused to join, was invaded, captured, and the men sold into slavery.
Now tradition realists would ignore the internal factors at play. Contemporary realists, like Richard Hanania understand that there are internal factors at play such as domestic politics, personality, culture, etc.
Realists are very skeptical of idealistic missions. The Bush plan to spread democracy across the Middle East was not realism. Wilson invading Mexico “to teach them to elect good men” was not realism. Even Vietnam was not realism, though many “realists” tried to justify it.
Nixon and Kissinger were realists. The biggest problem for realists is that they don't always see what their own national interests are, and therefore do things that are counterproductive.
The Athenians took a large expedition to Sicily. This was not realism at all. It was too far away and not in the Athenian sphere of influence. They thought it would hurt Sparta but it ended up costing Athens the war.
Overreach is real and empires need to focus on not making foolish mistakes. Realists are not perfect, but it seems more grounded in life as it is, instead of what it ought to be.