After centuries of general political stability, it is easy to imagine a sense of inevitability setting in; just as the seasons changed, so the Romans had their elections year after year, decade after decade, century after century. The Roman Republic with unusual regularity held elections, took votes, went to war, and passed laws. To be sure, there were disruptions occasionally—there were lawmakers and magistrates who overstayed their welcome; there was political strife, but as Tacitus writes, none of these lasted long and the Republic rolled on.

Until the Republic came to a sudden end and became an autocratic system ruled by one man, which modern historians refer to as the Roman Empire. Yet the Republic did not collapse all at once. There was a century of intermittent civil strife (roughly 133–31 BCE), which culminated in two decades of bloody civil war (49–31 BCE). At the end of the civil wars, Octavian, the adopted son of Julius Caesar, emerged victorious. He quickly assumed the name Augustus, The-August-One, and became Rome's first emperor.

Four hundred and sixty years are a pretty good run, and we may be just as inclined to ask why the Roman Republic lasted so long as to ask why it finally fell. Even in antiquity, Rome's success was a topic of reflection. The Greek historian and politician Polybius (202–120 BCE) wrote an entire history in an attempt to understand how Rome grew from a relatively powerful city in Italy to a Mediterranean-wide empire in such a short time—less than a hundred years. Polybius wrote at the height of the Republic and died just as it was seeing the first pangs of political tumult. He came to Rome as a high-profile political exile and became an astute observer of the Roman system. As a Greek, he brought an outsider's perceptivity and a knowledge of political theory.

In his search to understand Rome's success, Polybius found the answer in Rome's political institutions. Greek political theorists, beginning with Plato and Aristotle, had delineated the basic forms of political constitutions—monarchy, aristocracy, and democracy—and judged that the most durable and just constitution was one that mixed the best parts of each. In Rome, Polybius found in reality what had previously existed only in theory—a “mixed constitution.” He lays out how Rome combined elements from each constitution in its Republic.