The Roman Republic had no single ruler and no written constitution. Instead it balanced three kinds of power (magistrates, a council of elders, and the assemblies of the citizens) against one another.
The magistrates
Executive power ran through a ladder of annual offices, the cursus honorum. At the bottom were the quaestors, who managed finances; then the aediles, who ran the city's streets and games; then the praetors, who administered justice. At the top stood the two consuls, who led the armies and presided over the state. Every office was held in pairs or larger boards and for a single year only, so that colleagues could veto one another and no one accumulated lasting power. In a grave emergency the Romans could appoint a dictator with sweeping authority, but for six months at most.
The Senate
The Senate was a standing council of some three hundred men, mostly former magistrates who sat for life. Formally it only advised, but its advice carried immense weight: it controlled the treasury, directed foreign policy, and assigned commands. Its authority rested on the accumulated prestige of its members, and for centuries its word was, in practice, decisive.
The assemblies
Sovereignty, in theory, lay with the Roman people, who voted in assemblies to pass laws, elect magistrates, and decide war and peace. But voting was organised by group (by wealth class or by tribe) so that some citizens' votes counted for far more than others'. The system was republican, not democratic in the modern sense.
Checks and balances
The result was a machine of mutual restraint: consuls could veto each other, the Senate guided policy, the assemblies elected and legislated, and the tribunes (see the next lesson) could block almost anything. It was cumbersome, but its very friction kept any one man or faction from seizing control, until, in the Republic's last century, the balance failed.