Rome developed an army that enabled it to conquer the Mediterranean world, and governments that enabled it to rule for more than five centuries. Rome's system of law is one of its most important contributions to civilization. Today, Roman law forms one of the two major legal systems of the Western world.
The republic was governed by the Senate, various assemblies, and a number of officials. The Senate was an aristocratic body with many powers, the most important of which were the management of religious affairs, the conduct of foreign policy, and the supervision of revenue collection and expenditures. The Senate was open at first only to patricians, and throughout the history of the republic was controlled by great landowners.
The assemblies were intended to represent citizens other than the rich landowners. Only adult males were full citizens and could vote in assemblies. The earliest assemblies were the comitia curiata, which never held real power, and the comitia centuriata, which elected officials and enacted legislation. Both assemblies were controlled by patricians, and this fact led the plebeians to establish another assembly, the comitia plebis tributa; by 287 B.C. it had become the principal legislative body.
Officials of the Roman Republic had wide authority, but there were legal safeguards designed to prevent them from gaining too much personal power. The duties and authority of each office were divided among at least two men. Except for censors, they held office for one year only. A retiring official had to spend at least one year as a private citizen before holding another office. He could not hold the same office again for 10 years. However, during the last century of the republic, the laws on tenure and reelection were generally ignored.
The Roman emperors kept the forms of the republic for 200 years. Senate and assemblies continued to meet, and officials were regularly elected. For a while, the Senate ruled Italy and some of the provinces. The Senate was constitutionally responsible for selecting the emperor, but in most cases it, in effect, ratified a selection made by others, usually the army or the previous emperor. The emperor, backed by the army and assisted by a small group of senators and other advisers, ran the government.
Under the emperors, the army became immensely powerful. Augustus began the practice of stationing personal troops—the Praetorian Guard—in and around Rome. In 41 A.D. the Guard made Claudius emperor. Nero fell in 68 when Galba, the Roman commander in Spain, marched his army on Rome. New emperors began to reward the Praetorians with a donative, a sum of money to be given each soldier. At the death of Pertinax in 193 the troops were so sure of their power that they auctioned off the empire to the highest bidder. The Roman Empire had become a military monarchy.
The emperor Diocletian (284-305) made basic changes in the government. He took absolute command of all political and economic activities and had himself worshiped during his lifetime as a god. Diocletian ruled the eastern provinces and appointed a coemperor, Maximian, to rule the West. A divided government for the empire proved practical, and the separation into Eastern (Byzantine) and Western (Roman) division later became permanent.