Polybius: Hostage in Rome, Scipio's Tutor and Father of Checks and Balances

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Polybius was born around 200 BC in Megalopolis in Arcadia, son of Lycortas, a major landowner who became strategos, or commanding general, of the Achaean League. He traveled as an ambassador with his father in his 20s, and around 170 BC was elected hipparchus, the cavalry commander and second in command of the league. During the Third Macedonian War against the Antigonid dynasty, Lycortas urged neutrality. After Rome crushed Macedon at Pydna in 168 BC, it demanded 1,000 Achaean nobles as hostages in 167 BC, and Polybius was among them. In Rome, Lucius Aemilius Paullus, conqueror of Macedon, hired him to tutor his sons, including Scipio Aemilianus, and Polybius joined the Scipionic Circle, spending 17 years watching the Roman state from inside.

In 150 BC the roughly 300 surviving hostages were freed, but Polybius returned to Scipio and witnessed the destruction of Carthage in 146 BC. That same year Rome burned Corinth, and Polybius helped set up Roman-approved governments in the defeated Greek cities. His 40-book Histories, of which the first five survive intact, set out anacyclosis, a cycle from monarchy to tyranny, aristocracy to oligarchy, and democracy to ochlocracy, or mob rule. He argued that Rome's mixed constitution of consuls, Senate and popular assemblies let each check the others. He died at about 82 after falling from his horse. Machiavelli, Montesquieu and John Adams drew on his ideas, and his mixed constitution informed the U.S. Constitution.

• In his twelfth book he attacked the historian Timaeus as an armchair scholar and insisted historians need practical experience.
• He read Rome's treaties with Carthage in the archives and interviewed men who had known Hannibal.
• He favored his patron Scipio and despised the Aetolians.
• His five-by-five Polybius square turned each letter into row and column numbers that could be signaled with torches in each hand.
• The grid's logic carries into modern ciphers and prison tap codes.