Ancient Greek historian, c. 460-400 BC. Wrote History of the Peloponnesian War between Athens and Sparta. The first analytical historian, he developed a ruthlessly disillusioned theory of individual and state action as the amoral pursuit of power, in which morality was nothing but a pretence. Classic locus of this theory is the Melian Dialogue or Melian Debate, in which the Athenians threaten the Melians with destruction. Here's a snippet of a translation, although it can't do justice to his complex, serpentine prose style:

...

Melians: Then surely, if such hazards are taken by you to keep your empire and by your subjects to escape from it, we who are still free would show ourselves great cowards and weaklings if we failed to face everything that comes rather than submit to slavery.

...

Athenians: ...Do not be led astray by a false sense of honour - a thing which often brings men to ruin when they are faced with an obvious danger that somehow affects their pride. For in many cases men have still been able to see their dangers ahead of them, but this thing called dishonour, this word, by its force of seduction, has drawn them into a state where they have surrendered to an idea, while in fact they have fallen voluntarily into irrevocable disaster, in dishonour that is all the more dishonourable because it has come to them form their own folly rather than their misfortune....

[The Melians then decide to reject Athenian terms - peace in exchange for becoming part of the Athenian empire - and hope for Spartan aid to preserve their independence.]

...

Siege operations were now carried on vigorously and, as there was also some treachery from inside, the Melians surrendered unconditionally to the Athenians, who put to death all the men of military age whom they took, and sold the women and children as slaves. Melos itself they took over for themselves, sending out later a colony of 400 men.