The Roman Republic did not have a written constitution. Their system of government was based on custom and precedent. This makes it difficult to provide a detailed account of how things worked. At times powers shifted from one assembly to another, or multiple assemblies had partly the same powers. With that in mind, I'll try to give an overview of the Roman 'constitution' up to the first century BC (when civil wars made a mess of everything and Sulla tried to reform the constitution).

Consuls and praetors

(adapted from this post in the recent AMA)

The Roman Republic was organised in such a way that no one person could hold too much power. The Romans abhorred kings and absolute rulers. That is why, after the last king of Rome was deposed in the 6th century BC a system was set up where the power of the king (called imperium) was divided among two elected consuls, each having command of half the army and each having the authority to veto decisions of the other. To limit their power further, the consuls were elected for one-year terms. Additionally, it became customary that a former consul could not be elected as consul again for at least 10 years.

This worked fine while Rome was a city-state, fighting wars in the vicinity of the city. Every year the consuls could raise their armies, fight a war and return to Rome well before the end of the year. The army consisted of farmers, who be drafted for a single campaign season and would be back home again in time for harvest.

As Rome expanded, this system got more complicated. The consuls were regularly sent further from Rome and the number of citizens was growing, so a new magistracy was invented to reduce the workload of the consuls. In the fourth century BC one of the tasks of the consuls, the administration of justice, was given to a praetor. A praetor would also have imperium, but of a lesser degree than the imperium of consuls. Initially every year one praetor was elected, but in the third century this was increased to two praetors: one being responsible for Roman citizens (the Praetor Urbanus) and the other for non-citizens (the Praetor Inter Peregrinos). With the institution of the praetors, consuls would only have to concern themselves with justice in extraordinary cases.

The trick of using praetors to reduce the workload of consuls was used again in the third century BC, when Rome had gained control of Sardinia and Sicily. These islands were very important sources of grain, so Rome made them into the first provinces of the fledgling empire. Every year, two additional praetors (with lesser imperium) would be elected. They would be sent to Sardinia and Sicily respectively to basically govern those islands and to make sure the grain supply was secure.

By now every year two consuls and four praetors were elected, but still there was too much to be done. Wars were being fought farther from Rome and it was complicated to elect someone, send him overseas with an army, conduct a campaign season, and return home before the end of the year. Too much time would be lost. So another trick was devised. From the third century onwards a consul could be allowed to continue to hold imperium for another year. This is called prorogatio. Two new consuls would still be elected, but one or both of the previous consuls would be allowed to operate as proconsul ("acting in place of a consul"). This meant that he could continue whatever he was doing (usually fighting a war) while the new consuls would receive other tasks. Such an extension was always approved for a year, but might happen several years in a row. Later on the same would apply to praetors, allowing some to stay on as propraetor. With this ad-hoc system in place, Rome was able to field many armies across the western Mediterranean simultaneously.

So by the first century BC, under normal circumstances every year two consuls (with full imperium) and six praetors (with lesser imperium) would be appointed (six praetors because at this point an additional two were sent to Spain yearly). Additionally, the imperium of sitting consuls and proconsuls could be extended for a year, as well as the lesser imperium of praetors and propraetors. Though initially imperium was not tied to a geographical area, over time it did evolve to a point where someone was understood to have imperium not only for a limited time, but also over a limited area. This is the basis for the provinces governed by proconsuls and (pro-)praetors.

Other magistrates

Apart from consuls and praetors, the Romans had many other magistracies which I'll describe briefly here.

• Dictator

A dictator would be appointed in times of crisis. When it was determined necessary (by popular appeal or senatorial advice) a consul could name a dictator; the dictator would then take control of the state for a six month term. All other magistrates - including the consuls - were subordinate to the dictator, though the dictator's decisions were still subject to vetos by tribunes (see below). The dictator had consular imperium.

A famous example of a dictator being appointed during the Middle Republic was during the Second Punic War. Hannibal had defeated a Roman army in the north of Italy and at ambushed another army at Lake Trasimene, killing the consul who led that army. The Senate then appointed Quintus Fabius as dictator. After raising a new army Fabius joined the remaining consul Servilius, relieved him of command and merged the two armies to defend Rome against Hannibal. His policy of avoiding battle was unpopular, but it proved to be the right strategy to recover from the earlier disasters.

• Interrex

An interrex was appointed for five days if the administrative year started without a consul. If there still wasn't a consul after five days, another interrex would be chosen. The interrex had consular imperium.

• Tribune of the plebs

There were ten tribunes of the plebs elected every year. The tribunes of the plebs assisted the common people in disputes with magistrates and the upper class. Tribunes were sacrosanct, meaning the could not be harmed in any way. They had the right veto any magistrate's decision, any law or decree, and any election. Tribunes could propose bills and hold trials. After their time in office they would be granted membership of the Senate (if they weren't a member yet).

• Quaestor

There were 8 or more quaestors appointed every year. Two of them were in charge of the treasury of Rome, the others performed various administrative duties in Italy and for commanders in the field. After serving as a quaestor one was granted membership of the Senate.

• Censor

Censors were unique in that they were elected every five years and held office for 18 months. Two censors would be elected. Their tasks were: conducting a census of citizens; making up the lists of senators and equites ('knights'); giving out contracts for a broad range of activities.

• Aediles

Four aediles supervised certain public buildings and places. They also put on games for Rome's inhabitants.

The Senate

(adapted from this post in the recent AMA)

The Senate was for much of the Republic the most influential of the Roman institutions. Formally their only duty was to provide advice to the magistrates and to give their opinion on pending legislation. Of course as experienced statesmen their opinion carried weight, and they weren't above bribing people to make their opinion carry some more weight. Aside from these traditional duties, in practice the Senate had usurped some duties from the popular assemblies (this was possible because Rome didn't have a written constitution). The Senate had a final say in state finances, the granting of triumphs and extentions of imperium (prorogatio). Furthermore, foreign policy was considered to be the domain of the Senate. Treaties had to be approved by the Senate, though the declaration of war still had to be approved by a popular assembly.

All senators were wealthy and membership was limited to ex-magistrates. Most senators came from a long line of senators (the patricians). It was possible for plebeians (non-patricians) to join the Senate, if they managed to secure a magistracy such as the consulship. By law each year at least one of the consuls had to be plebeian. What happened here though is that after a while the plebeians who had managed to join the Senate closed ranks and made sure all newly elected plebeian magistrates came from their own group of friends and family. So the Senate consisted of patricians and plebeians, but all had in common that they were very wealthy, had plenty of political experience, and tried to stick to the power.

So all magistrates would be joining the Senate at some point in the future, if they weren't already in the Senate. This is one reason why the Senate had such huge influence on magistrates. If the Senate 'advises' a magistrate to do something, not listening could lead to problems later on. Even if the magistrate has the authority to do as he pleases at that moment, he'll be an ex-magistrate as soon as the year is over. At that point he's at the mercy of his peers. Crossing the Senate might mean no more lucrative jobs. But this went both ways. The Senate actually often gave magistrates considerable leeway in doing what they wanted, especially overseas. After all, if the current magistrate gets such leeway, the next one - which would likely be someone else from the senatorial class - would also get such leeway.

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