Thucydides
The failed Athenian general whose account of the Peloponnesian War invented the idea that politics between states can be explained rather than moralized.
Thucydides was an Athenian aristocrat who was given a fleet command in 424 BC, failed to relieve the city of Amphipolis in time, and was exiled for it. The exile is the reason we have his book. Barred from Athens for twenty years and able to travel among the enemy, he spent the war interviewing participants on both sides and writing the History of the Peloponnesian War, which breaks off unfinished in 411 BC.
He is usually called the first scientific historian, which undersells him. What he actually invented was a way of looking at politics: the assumption that the behavior of states follows from their situations, that stated reasons and real reasons differ, and that the analyst's job is to find the second underneath the first.
The method
Herodotus, writing a generation earlier, had explained events partly through oracles, dreams, and divine jealousy. Thucydides removed all of it. His plague at Athens is described clinically, with symptoms in order, and he notes that prayer and medicine were equally useless. His stated aim was to write something useful for people who want to understand events that, human nature being what it is, will happen again in similar form.
He also invented the analytical distinction that political scientists still use. The Spartans and Athenians each gave a list of grievances for going to war, disputes over Corcyra, Potidaea, a trade embargo. Thucydides sets these out and then says they were pretexts. The truest cause, he writes, the one least openly stated, was that the growth of Athenian power frightened the Spartans and made war inevitable.
That sentence is the origin of the structural explanation of war: not that anyone chose it, but that a shift in relative power created a situation neither side could exit.
The Melian Dialogue
The most quoted passage in the book, and the most disturbing, is the exchange he stages in 416 BC between Athenian envoys and the leaders of Melos, a small neutral island Athens intends to absorb. The Melians appeal to justice, to the gods, to their kinship with Sparta. The Athenians decline to argue about justice at all, on the grounds that it only arises between equals in power, and state the position plainly: the strong do what they can and the weak suffer what they must.
The Melians refuse. Athens takes the island, kills the men of military age, and enslaves the women and children.
What Thucydides thinks of this is deliberately left out, which is why the passage has been claimed by everyone. Realists read it as the truth about international politics stated without hypocrisy. Others read it as an indictment, noting that Athens goes on to launch the catastrophic Sicilian expedition in the very next book and destroys itself. Both readings are defensible, and the ambiguity is the point: he shows you the logic and its consequences and declines to preach.
The echo
Thucydides is the founding text of realism in international relations, and every serious writer in that tradition, from Hobbes, who translated him, to Hans Morgenthau and Kenneth Waltz, works in his shadow.
He is also, unfortunately, the source of a modern catchphrase. Graham Allison's Thucydides Trap, the idea that a rising power and an established power are structurally likely to fight, takes its name from the sentence about Athenian growth and Spartan fear and is now applied constantly to China and the United States. Whether the pattern holds is genuinely disputed, and Thucydides himself said inevitable about one war, not about a law of history.
The more durable thing he left is the habit rather than the theory. Ask what a state's situation is rather than what it says. Assume the stated cause is not the whole cause. Report what the ugly side actually argued, in its own words, rather than what it should have argued. Twenty-four centuries later that is still what good analysis looks like.