Caesar's assassination brought anarchy, out of which the Second Triumvirate emerged with the rule of Octavian (later Augustus), Antony, and Lepidus. Octavian was Caesar's nephew, ward, and heir, and his true successor. At Actium (31 B.C.) he defeated Antony and Cleopatra and made the empire one. No change was made in the government, but Octavian received from the senate the title Augustus and from the people life tribuneship; this, with the governorship of all the provinces conferred by the senate, made him the real ruler. He was called imperator [commander] and princeps [leader] and is usually considered the first Roman emperor. (For a list of the Roman Emperors from Augustus to the fall of Rome and the years they reigned, see the table entitled Rulers of the Roman Empire.)
Augustus organized provincial government and the army, rebuilt Rome, and patronized the arts and letters. His rule began a long period (200 years) of peace, called the Pax Romana. During this time the Roman Empire was the largest it would ever be; its boundaries included Armenia, middle Mesopotamia, the Arabian desert, the Red Sea, Nubia, the Sahara, the Moroccan mountain mass, the Atlantic Ocean, the Irish Sea, Scotland, the North Sea, the Rhine, the Danube, the Black Sea, and the Caucasus. Augustus' chief additions to the empire were a strip along the North Sea W of the Elbe and part of the Danubian area.
The blessings of peace were great for the empire. The extensive system of Roman roads made transportation easier than it was again to be until the development of railroads. A postal service was developed closely tied in with the organization of the army. Commerce and industry were greatly developed, particularly by sea, over which grain ships carried food for Rome and the West from the ports of northern Africa. The Roman Empire became under Augustus one great nation. The enlarged view of the world made a great impression on Rome, where literary and artistic interests were of importance, although nearly always tending to imitation of Greece and of the East.
Augustus died A.D. 14 and was succeeded by his stepson Tiberius; his general Germanicus Caesar fought fruitlessly in Germany. Caligula, who followed, was a cruel tyrant (A.D. 37–A.D. 41); he was succeeded by Claudius I (A.D. 41–A.D. 54), who was dominated by his wives, but during his rule half of Britain was conquered (A.D. 43). In his time Thrace, Lydia, and Judaea were made Roman provinces. His stepson Nero (A.D. 54–A.D. 68) was an unparalleled tyrant. In his reign occurred the great fire of Rome (A.D. 64), attributed (probably falsely) to Nero; it burnt everything between the Caelian, the Palatine, and the Esquiline, but it was a boon to the city, for Nero moved the population to the right bank of the Tiber, then very thinly populated, and rebuilt the region with broader streets and great buildings.
At that time an entirely new element, Christianity, made itself felt in Rome. On Nero's orders a barbarous persecution took place in which many Christians died, among them St. Peter and St. Paul. Throughout the Roman Empire the Christians expanded steadily for the next centuries. Their conflict with the empire, which brought on them continual persecution, was chiefly a result of the Christian refusal to offer divine honors to the emperors. But Christianity penetrated the army and the royal household in spite of the constant danger of detection and persecution. There were many periods in the first three centuries when Christians worshiped openly, even in Rome, where the catacombs housed not only graves but also churches.
With Nero the Julio-Claudian line ended. There was a brief struggle (see Galba; Otho; Vitellius) before Vespasian (A.D. 69–A.D. 79) became emperor. Under him his son Titus destroyed Jerusalem (A.D. 70); Titus then briefly succeeded his father. After his mild, rather benign rule, his brother Domitian (A.D. 81–A.D. 96), a despot and persecutor of Christians, gained the empire. In Domitian's reign Agricola conquered Britain almost entirely. Domitian was unsuccessful in his dealings with the Daci and finally bought them off. After Nerva came Trajan (A.D. 98–A.D. 117), one of the greatest of emperors. Trajan undertook great public works, defeated the Daci and established Roman colonies there (in what is now modern Romania), and pushed the eastern borders past Armenia and Mesopotamia.
Trajan's successor, Hadrian, withdrew Roman rule to the Euphrates and in Britain built his wall (Hadrian's Wall) to hold back the barbarians who constantly threatened that fast-developing province. He also reorganized the senate and the army. Roman armies were then seldom seen far from the boundaries of the empire, and life continued throughout the Roman world in peace and quiet. Italy was sinking into a purely provincial state, although many emperors made attempts to make it a special country. The successors of Hadrian were Antoninus Pius (138–161) and Marcus Aurelius (161–180), who ruled in what is commonly called the Golden Age of the empire.
With Commodus (180–192) the decline of the empire is usually said to have begun. The age of the Praetorians was then at hand, when the rise and fall of emperors was determined by this elite corps of soldiers. Septimius Severus (193–211) was unusually able for his period; he campaigned with success against the Parthians and against the Picts of N Britain. His son Caracalla is noteworthy for extending Roman citizenship to all free men of the empire and for the famous baths named after him.
Emperors succeeded one another rapidly in the 3d cent.: Heliogabalus, Alexander Severus, Philip (Philip the Arabian), and Decius among them. Decius was one of the most violent persecutors of Christians; he fell fighting the Goths, first of the Germans, who were eventually to overwhelm the empire. In 260 the emperor Valerian was captured by the Persians, and the empire fell into anarchy. The provinces suffered from increasingly bad government as well as from a pestilence that carried off half the population. Claudius II (268–70) revived Roman fortunes somewhat, while Aurelian (270–75) overthrew the Palmyrene kingdom of Zenobia.
In 284, Diocletian was made emperor by the army. He was a reformer of government and of the social order, but only one of his efforts was successful. This was the division of the empire into four political sections, two eastern and two western. There were to be two Augusti and two Caesars.
The division of East and West was resumed after the death (337) of Constantine I, who moved the capital to Byzantium, renamed Constantinople. By the Edict of Milan (313), Constantine granted universal religious tolerance, thus placing Christianity on the same footing as the other religions. He divided the empire administratively into prefectures, dioceses, and provinces; the bishops thus gained great influence and shared in the authority of the civil administration. There was a brief resurgence of paganism under Julian the Apostate, but Christianity was securely established.
On the death of Jovian, Julian's successor, Valentinian I (364–75) ruled the Western Empire; Valentinian II (375–92) succeeded him. After the death (395) of Theodosius I the empire was permanently divided into Eastern (see Byzantine Empire) and Western, and Rome rapidly lost its political importance.
Under the emperors, Rome had been the center of the world. It must have presented a splendid, although heterogeneous, appearance. Little remained of the original city, for the emperors had replanned it to glorify themselves as well as the city. Parts of the Aurelian Wall still stand. On the Capitoline were the citadel (the arx) and the temple of Jupiter Capitolinus; the Palatine was the site of the palaces of Augustus and Tiberius (the word palace derives from the hill); the palace of Nero and Trajan's baths were on the southern slopes of the Esquiline. South of the Palatine was the Circus Maximus, where the famous chariot races were held. The old Roman Forum (see forum), extending from the Palatine almost to the Colosseum, remained the center of the city; northwest of it were the Emperors' Fora, with many fine public buildings, and the Temple of Peace. On the Martian Field were Pompey's theater, the Circus Flaminius, the Pantheon (see under pantheon), and the baths of Agrippa and Nero. Across the Tiber was Nero's circus, where St. Peter's now stands; Hadrian's tomb, now known as the Castel Sant' Angelo, has survived as a major landmark. The largest of the many public baths were those of Caracalla, near the Appian Way.
At its height, imperial Rome counted well over a million inhabitants. It was well policed, sanitation was excellent, and a fire-fighting force of seven brigades was maintained. Nineteen imposing aqueducts, of which many remains are extant, supplied the city with water. Among the rich such luxuries as central heating and running water were not unknown. The indigent (c.200,000) were cared for at public expense. Not until the 18th cent. were luxury and technical proficiency on a comparable scale to return to any European city.
Decline, once it began, came quickly, however. Honorius (395–423) made Ravenna the capital of the West; other emperors chose Milan and Trier (Treves), where they were nearer the border to check Germanic attacks. The West sank into anarchy, and Italy was ravaged by invaders. Alaric I took Rome in 410, and Gaiseric conquered it in 455. Attila was kept from sacking it, supposedly through the efforts of the pope, Leo I (St. Leo the Great). In this general disintegration the popes, originally the bishops of Rome, greatly increased their power and prestige, thus restoring to Rome in the religious field the importance it had lost in the political.
In 476 the last emperor of the West, appropriately called Romulus Augustulus, was deposed by the Goths under Odoacer; this date is commonly accepted as the end of the West Roman Empire, or Western Empire. The so-called Dark Ages (now usually called the Early Middle Ages; see Middle Ages) that followed in Western Europe could not eradicate the profound imprint left by the Roman civilization. Roman law is still alive; the Romance languages are but modifications of Roman speech. Roman Catholicism for nearly 12 centuries was the only religion and the main cultural force of Western Europe.
The fall of Rome marked no abrupt ending of an era, for the barbarians that filled the gap left by the disappearance of the old order were quick in accepting and adapting what vital elements remained of it. The survival of the East Roman Empire, or Eastern Empire, and the creation of the Holy Roman Empire showed how much vitality was left in the imperial ideal. Italy itself, however, did not recover from the fall of Rome until the 19th cent.