Polybius

Polybius was a Greek statesman of the Achaean League taken to Rome as a hostage in 167 BCE, where he became the friend and adviser of Scipio Aemilianus and stood beside him at the destruction of Carthage. The Histories, in forty books of which five survive complete, explain how Rome conquered the known world in fifty-three years, and the sixth book, on the Roman constitution as a mixture of monarchy, aristocracy and democracy that checks each element by the others, was read by Machiavelli, Montesquieu and the American founders as the theory behind the separation of powers. He wrote history for the use of statesmen and insisted on seeing the ground himself.