Polybius

How unusual, that a Greek aristocrat, captured and enslaved, would write a book praising the city and empire into which he was taken - yet that is precisely what Polybius does, and in the process offers us a detailed description of the Roman government. He outlines the three branches of the Republican government, and tells us what each one does, and how they balance each other in terms of power.

He also offers a prediction of the forces which will ultimately bring down the Republic. Internal issues, not external attacks, will be the end of this structure.