Roman Emperors: From Augustus to the Fall of Rome

For roughly five centuries, from 27 BCE to 476 CE in the West and until 1453 CE in the East, the Roman world was ruled by emperors. Some were brilliant generals, some were philosophers, some were madmen, and a shocking number were assassinated in their beds before they had time to warm the purple. Their reigns determined the fate of an empire that stretched from the Scottish Highlands to the Syrian desert, and their decisions — to persecute Christians, to move the capital, to split the empire in two, to hire Germanic mercenaries — shaped the world we live in.

This article is a comprehensive overview. It explains the office of the Roman emperor, traces the great dynasties from the Julio-Claudians to the Flavians to the Five Good Emperors, follows the crisis of the third century and the Tetrarchy of Diocletian, follows the conversion of Constantine and the eventual collapse of the Western Empire, and introduces the major individual emperors in their own dedicated biographies.

The Invention of the Principate

The office of Roman emperor was never formally created. Augustus, the first to hold it, was careful to maintain the fiction that he was merely the princeps, the “first citizen,” of the Roman Republic. In 27 BCE the Senate granted him the title Augustus and the honorific imperator, but he held no position that the Republic had not known. He was consul, tribune, and proconsul. He commanded the armies. He was, in the words of the historian Tacitus, “subtle and patient” in accumulating power without ever appearing to claim it.

In reality, Augustus held a unique bundle of powers. As tribune, he could propose legislation to the popular assemblies and veto any act of the Senate. As proconsul, he commanded the military districts where every legion was stationed. He controlled the grain supply, the mint, the appointment of provincial governors, and foreign policy. By the time of his death in 14 CE, the Republic was dead in all but name, and his successors would rule as absolute monarchs.

The Julio-Claudian Dynasty (27 BCE – 68 CE)

The first dynasty of emperors, the Julio-Claudians, ruled for almost a century. Augustus, Tiberius, Caligula, Claudius, and Nero were bound by blood and adoption, descending either from the Julian line of Julius Caesar or the Claudian line of Augustus’s wife Livia.

Augustus was the ablest of them, a careful administrator who stabilized the empire after a century of civil war. Tiberius was a competent general who ruled well in the first half of his reign and degenerated into paranoia and cruelty in the second. Caligula, his successor, ruled for less than four years and is remembered for his cruelty, his extravagance, and his apparent madness. Claudius, the unlikely scholar who had been kept out of public life for decades because of a physical disability, proved a surprisingly effective emperor who conquered Britain. Nero, the last of the dynasty, murdered his mother and his wife, persecuted Christians after the great fire of Rome in 64 CE, and was forced to commit suicide in 68 CE.

The Year of Four Emperors and the Flavians

Nero’s death triggered a civil war in 69 CE, the Year of the Four Emperors. Galba, Otho, and Vitellius all claimed the throne in quick succession. The eventual victor was Vespasian, the unflappable general who had been besieging Jerusalem during the entire crisis. Vespasian founded the Flavian dynasty and began the construction of the Colosseum, the largest amphitheater ever built in the Roman world. His son Titus completed the Colosseum and also completed the destruction of Jerusalem in 70 CE, an event commemorated on the Arch of Titus that still stands in Rome. The third Flavian emperor, Domitian, ruled as a tyrant and was assassinated in 96 CE.

The Adoptive Emperors and the High Point of the Empire

The assassination of Domitian ended the Flavian dynasty and opened the way to what the senator-historian Tacitus considered a happier age. The Senate appointed Nerva as emperor in 96 CE, and Nerva adopted the general Trajan as his heir. This pattern of adoptive succession — choosing the best man rather than the biological son — produced four more emperors in succession: Trajan, Hadrian, Antoninus Pius, and Marcus Aurelius. Together with Nerva, they are known as the Five Good Emperors.

Trajan conquered Dacia and reached the Persian Gulf, taking Rome to its greatest territorial extent. Hadrian consolidated the empire, building Hadrian’s Wall in Britain, the Pantheon in Rome, and a network of roads and forts across the provinces. Antoninus Pius ruled during a long, prosperous, and largely peaceful reign. Marcus Aurelius, the philosopher-emperor, spent most of his reign on the Danube fighting the Marcomannic Wars, and his Meditations, written in his tent, are still read today as one of the great works of Stoic philosophy.

The Crisis of the Third Century (235–284 CE)

The death of Marcus Aurelius in 180 CE, followed by the assassination of his son Commodus in 192 CE, ended the adoptive system. The third century saw at least 26 emperors, most of them army generals raised by their troops and killed by their rivals. The empire nearly collapsed under the combined pressure of barbarian invasions, plague, and economic crisis. For a full discussion, see the article on the Crisis of the Third Century.

The crisis was eventually resolved by Diocletian, who took the purple in 284 CE and ruled for twenty years. Diocletian reorganized the empire into the Tetrarchy, a system of four co-emperors, two Augusti and two Caesars, intended to provide stable succession and to defend the vast frontiers. He also initiated the last great persecution of Christians, beginning in 303 CE.

Constantine and the Christian Empire

The tetrarchic system did not survive Diocletian’s abdication in 305 CE. A new round of civil wars followed, ending with the victory of Constantine the Great, the son of one of Diocletian’s Caesars. Constantine’s reign, from 306 to 337 CE, transformed the Roman world. He legalized Christianity with the Edict of Milan in 313 CE, convened the Council of Nicaea in 325 CE, and moved the capital of the empire from Rome to the old Greek city of Byzantium, which he renamed Constantinople and which would remain the capital of the Eastern Roman Empire for more than a thousand years.

Constantine’s dynasty, the Constantinian dynasty, ended with the death of his son Constantius II in 361 CE. The brief restoration of paganism under Julian the Apostate was reversed by his successors, and by the end of the fourth century Christianity was the official religion of the empire.

Theodosius and the Division of the Empire

Theodosius I, who ruled from 379 to 395 CE, was the last emperor to rule a united Roman Empire. He made Christianity the state religion and closed the ancient pagan temples. When he died in 395 CE, he divided the empire between his two sons: Arcadius in the East, Honorius in the West. The division was intended to be temporary; it proved permanent.

The Fall of the Western Empire

The fifth century was the slow disintegration of the Western Roman Empire. Theodosius had relied on Gothic mercenaries to win his battles, and those mercenaries, especially the Visigoths under Alaric, soon turned on their employers. The Visigoths sacked Rome in 410 CE. The Vandals, having crossed from Spain to North Africa in 429 CE, sacked Rome again in 455 CE. The last emperor of the West, Romulus Augustulus, was deposed by the Germanic general Odoacer in 476 CE. For the full story, see the article on the fall of the Western Roman Empire.

The Survival of the Eastern Empire

The Eastern Roman Empire, ruled from Constantinople, survived for almost another thousand years. The citizens of this state never called themselves Byzantines; they called themselves Romans, and their empire was, in their view, the continuing Roman Empire. It would not fall until 1453, when the Ottoman Turks under Sultan Mehmed II breached the walls of Constantinople and killed the last Roman emperor, Constantine XI.

The Eastern Empire preserved Roman law, Roman administration, Roman engineering, and the Orthodox Christian Church. For most of its history, its emperor was the most powerful ruler in the Christian world. Even at its lowest point, after the Arab conquests of the seventh century stripped it of its southern provinces, Constantinople remained the largest and wealthiest city in Europe.

The Legacy of the Emperors

The Roman emperor became the model for almost every later European monarch. The Holy Roman Emperors of the Middle Ages, the Tsars of Russia, the Habsburg rulers of Austria, and Napoleon all claimed to be successors, in some sense, of Augustus and Constantine. The very word “emperor” derives from the Latin imperator, the title that Augustus’s successors inherited from the Republic.

The lives of the emperors are still studied as case studies in the exercise of absolute power. Augustus shows how to consolidate a revolution. Tiberius shows what happens when an aging ruler withdraws from public life. Caligula and Nero show the dangers of an emperor who believes he is a god. Trajan and Marcus Aurelius show the ideal of the philosopher-king. Diocletian shows how to reorganize a state in crisis. Constantine shows how a single ruler can transform a civilization by adopting a new religion.

Emperors and the Army

The defining relationship of Roman political life was between the emperor and the Roman army. Unlike modern rulers, who typically control the military through a chain of command, the Roman emperor had to maintain a personal, direct, and continuous relationship with the men who made him. The imperial succession was rarely determined by inheritance alone. It was determined, more often than not, by the decision of the legions, the Praetorian Guard, and the imperial frontier armies to acclaim a particular candidate.

The mechanics of this relationship were elaborate. The emperor was imperator, the supreme commander of every legion. He appointed every legionary legate, paid every soldier, and was responsible for every donative. When a new emperor was proclaimed, he was expected to travel to the armies and confirm his elevation in person, and the troops expected an immediate cash payment in exchange for their loyalty. The amount grew steadily: a single aureus per soldier under Augustus, five aurei under later emperors, and much more in times of crisis. The sums involved were enormous. When Septimius Severus came to power in 193 CE, he had to pay his Danubian legions so heavily that he had to seize the property of the Roman aristocracy to do it.

The emperor’s relationship with the army was the source of his greatest strength and his greatest danger. The army could make an emperor: Vespasian was proclaimed by the eastern legions in 69 CE while still besieging Jerusalem. The army could also unmake him. The third century was a parade of military emperors, raised by their troops and killed by their rivals, in a chaos that only ended with the accession of Diocletian. The annual average tenure of an emperor in the third century was less than three years.

The emperors who succeeded in ruling for long periods were those who mastered the art of managing the army. Augustus set the pattern: he paid the soldiers, settled the veterans, kept the legions in their permanent forts, and avoided asking the Senate to vote on military matters. The good emperors of the second century, from Nerva to Marcus Aurelius, were the heirs of this tradition. The bad emperors were those who lost the trust of the army: Domitian was assassinated by his own courtiers, Commodus was strangled by a wrestler in his bath, and Elagabalus was murdered by the Praetorians when he tried to elevate a Syrian sun god above Jupiter.

A particular feature of the late Empire was the development of the comitatus, the mobile field army that accompanied the emperor on his campaigns. Constantine’s reforms, in particular, separated the comitatenses, the mobile field armies, from the limitanei, the static frontier troops, and created a system in which the field army was the emperor’s personal instrument, well paid and politically reliable. The system survived into the Byzantine Empire and underlay the success of the great eastern emperors from Theodosius to Heraclius.

The Imperial Court and Daily Life of the Palace

The Roman emperor was a figure of absolute power, but the day-to-day governance of the empire was conducted through an elaborate bureaucracy centered on the imperial court. The court of Augustus was still small enough to fit in a private house on the Palatine Hill. The court of Diocletian or Constantine was a vast administrative machine with thousands of officials, an imperial chancery, a postal system, and a network of agents that extended to every province.

The imperial household was organized on the model of a Roman aristocratic family, but on a vastly expanded scale. The Praefectus Praetorio, the Praetorian Prefect, was originally the commander of the Praetorian Guard but became, by the third century, the chief minister of state, with both military and civil responsibilities. The Magister Officiorum, the Master of Offices, was the head of the imperial chancery and controlled the secret service, the foreign embassies, and the imperial courier system. The Quaestor Sacri Palatii, the Quaestor of the Sacred Palace, drafted the imperial laws. Beneath these great officials stood a hierarchy of secretaries, notaries, and clerks who ran the imperial administration.

The day of an emperor was a public performance. The emperor received petitions, heard cases, and issued rulings in the consistorium, the imperial audience chamber. He held formal dinners, the triclinia, in which the seating was carefully arranged to reflect rank. He reviewed the troops, attended the games, and participated in the major religious festivals. The emperor was expected to be visible, accessible, and generous, but also aloof, dignified, and unapproachable. The ritual of the court was designed to remind everyone, including the emperor, of his elevated status.

The sacred bedchamber, the cubiculum, was the innermost circle of the court, where the emperor was attended by the eunuch chamberlains. The eunuch, a category of official largely absent from the early Empire, became increasingly prominent from the fourth century onward, especially in the Eastern Empire. Men like Eutropius under Arcadius, Chrysaphius under Theodosius II, and Narses under Justinian wielded enormous power. The system was widely criticized in its own time, but it was also remarkably efficient. The eunuch had no family to enrich, no heirs to provide for, and no personal political base, and these features made him, in the eyes of many emperors, more reliable than any uncastrated official.

The imperial family itself was a political institution of the first order. The emperor’s wife, the Augusta, often wielded considerable influence: Livia, the wife of Augustus, was effectively co-ruler for the last decades of her husband’s reign. Agrippina, the mother of Nero, exercised power as regent for the first years of her son’s reign. Helena, the mother of Constantine, undertook the famous pilgrimage to the Holy Land that led to the discovery of the True Cross. The emperor’s sons were often raised to the rank of Caesar in their fathers’ lifetimes, given a junior military command, and expected to be the heirs. The system was designed to ensure smooth succession; in practice, it more often produced civil war.

The Imperial Cult and the Divinity of the Emperor

One of the most distinctive features of the Roman Empire was the imperial cult, the practice of worshipping the reigning emperor as a divine figure. The cult was not, at first, a sign of megalomania. It grew out of the older practice of worshipping the genius, the divine spirit, of the Roman state and its leaders. The first Roman to be formally deified was Romulus, the legendary founder of the city, who was said to have been carried up to heaven in a thunderstorm. Julius Caesar was formally deified after his death, and his adopted son Augustus was careful to refuse any living cult in Rome itself while permitting one in the eastern provinces, where Greek ideas about the divinity of rulers were already strong.

The cult took different forms in different parts of the empire. In Rome and Italy, the emperor was worshipped as the genius of the Roman state, and the formal priesthood of the cult, the Sodales Augustales, was staffed by senators and equestrians. In the eastern provinces, the emperor was often worshipped in temples, with altars, priests, and a regular cult calendar, very much as the local gods were worshipped. The provincial assembly of Asia, for example, organized a regular festival in honor of Augustus, with games, sacrifices, and athletic contests.

The cult was useful for the empire in several ways. It reminded subjects that the emperor was a single, universal figure, present in every province and worshipped by all peoples. It provided a common focus for the otherwise diverse religious life of the empire. And it provided a vocabulary for the emperor’s relationship with the gods: an emperor who was himself divine could be a priest on behalf of the Roman people, and the Pontifex Maximus, the title that the emperors inherited from the Republican pontiffs, was the most important of all.

The cult became increasingly elaborate over time. Caligula, who went further than any of his predecessors, ordered his statue to be placed in the Temple of Jerusalem, an act of breathtaking provocation that contributed to the Jewish revolt of 66 CE. Domitian demanded to be addressed as Dominus et Deus, “Lord and God.” The deification of the dead emperor became a regular feature of imperial succession, and most emperors were formally deified by the Senate after their deaths, even those who were deeply unpopular in life. The cult outlived the empire itself, transformed in the Christian East into the veneration of the emperor as the chosen of God, and in the medieval West into the ideology of the Holy Roman Emperor as God’s deputy on earth.

The cult also created one of the great religious conflicts of the early Empire. Christians refused to sacrifice to the genius of the emperor on the grounds that this was idolatry, and they were periodically persecuted for this refusal. The persecutions of Nero, Domitian, Marcus Aurelius, Septimius Severus, and finally Diocletian were all, in part, responses to Christian refusal to participate in the imperial cult. The conflict was only resolved by Constantine, who himself became a Christian and made Christianity the favored religion of the state, ending the imperial cult in its ancient form. The Christian emperor, however, inherited much of the imperial cult’s vocabulary, and the Byzantine emperor was worshipped in his own right as God’s vicegerent on earth for almost a thousand years. For the broader history of imperial succession, see the article on how Roman emperors were chosen.