Polybius was born into a rich and influential family in Megalopolis, in the Greek state of Arcadia, in 200 BC. By the time he was 30 he was a cavalry commander in the army of the Achaean Confederation (headed by Athens). Although he was a man of action, he had also by that time made his mark as a biographer and as an author on military tactics. Polybius was one of several Achaean commanders who offered military support to Rome in its war against Perseus of Macedonia. But Rome didn't trust the Achaeans, and, even after defeating Perseus at Pydna in 168, the Romans took 1000 eminent Achaeans, including Polybius, back to Rome as hostages for Greek good behavior.
Once in Rome, Polybius attracted the attention of the great Roman General, Scipio Aemilianus (Africanus the Younger), and Polybius began his Roman advancement under the protection of Scipio. It is fairly certain the Polybius went with the general on his campaigns in Spain and North Africa against the Carthaginians, and he surely was present at the destruction of Carthage by Scipio Aemilianus in 146 BC. Polybius later wrote what is still considered to be the definitive history of the Punic wars.
Meanwhile Roman distrust of the Greeks had proved well founded. Rome had to put down an Achaean revolt in 146 BC, and Polybius was designated by both sides to work out the post-revolt settlement. Polybius did well for his native Achaea getting a very reasonable settlement. For his efforts he was considered a hero by his countrymen -- statues and laudatory inscriptions dedicated to him can still be found in several parts of Greece.
As part of his effort to induce the Achaeans to thereafter stay on the side of Rome, Polybius wrote a forty-volume history of the rise of the Roman Republic to international power in the preceding 50 or so years. Most of that work was lost, but the first five volumes and parts of others did survive. The part of Book six that was preserved included his theories on the benefits of "complex constitutions" for states. His exemplar was the constitution of Republican Rome.
Those remaining volumes were to become the textbook for governmental theory for the European "Enlightenment" of the 18th century. And here's where the U.S. connection comes in. "Complex constitutions", if you hadn't already guessed, are systems of government organization that set up executive, legislative, and judicial branches of government whose powers must be checked and balanced against each other. This was the only way to avoid the dangers inherent in any of the three possible "simple constitutions", which, according to Polybius, were always unstable: monarchy, he said, always degenerates into tyranny, aristocracy always becomes plutocracy, and pure democracy, because it does not protect minorities, always degrades into mob rule.
The framers of the U.S. constitution knew all about Polybius, and they consciously built his theories into the U.S. Constitution of 1789. The theories of Polybius were a serious topic of discussion at the Constitutional Convention. Benjamin Franklin, the first U.S. diplomat and the U.S. Ambassador to France, shipped back many copies of translations of Polybius to Convention participants. The theories of the 18th-century French philosopher and political historian Montesquieu were also heavily debated and were eventually adopted, but Montesquieu, in his writings, acknowledged his own debt to Polybius.
European, Latin American, and African republican constitutions that came later all, to some extent, copied the American Constitution and Polybius.
Polybius and the Founding Fathers -- the separation of powers:
Polybius, works and other links:
The sixth book of Polybius' history:
The Roman Republican government (a modern analysis):