Polybius and the Separation of Powers.

Why was the Roman Republic so successful? Writing in the second century BCE, the Greek historian Polybius believed that much of Rome's strength came from its unusual political system. He described a mixed constitution combining three forms of government: the consuls represented monarchy, the Senate aristocracy, and the popular assemblies democracy. Each institution possessed powers that the others needed, preventing any one of them from easily dominating the republic.

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Polybius was particularly concerned with how governments could deteriorate. Monarchy might become tyranny, aristocracy might become oligarchy, and democracy could descend into what he regarded as mob rule. Rome's mixed constitution offered a way to resist these risks. Consider a consul commanding an army. Although consuls exercised considerable military authority, they depended on the Senate for funding and supplies. Even a victorious commander could later face prosecution by the people for misconduct during his term. The consul's power was considerable, but it was not unlimited.

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The Senate, meanwhile, managed finances and foreign relations and advised magistrates, while the popular assemblies elected officials, passed laws, and exercised certain judicial powers. These institutions were not equally representative of Roman society. Wealthy citizens enjoyed disproportionate influence in some assemblies, and aristocratic families dominated much of Roman politics. Nevertheless, Polybius admired the way the different parts of the constitution depended on one another. His account would later become an important reference for political thinkers interested in balanced government, including John Adams.