Your guide to the Roman empire: when it was formed, why it split and how it failed, plus its most colourful emperors

The transition from republic to empire was due in large part to the assassination of Julius Caesar in 44 BC. Along with Crassus and Pompey, Julius Caesar was one of the First Triumvirate that ruled the late Republic, but after the former’s death and the latter’s defeat in a civil war, he took sole control. He was eventually declared dictator perpetuo, or ‘dictator for life’. It turned out to be a hollow title, for his life was ended a little over a month later, brutally curtailed by dagger-wielding senators eager to uphold Rome’s republican ideals.

Caesar was succeeded by a fresh triumvirate, consisting of Mark Antony, Lepidus and Octavian, named in Caesar’s will as his adopted son and heir. Octavian thus saw himself as the rightful, single leader. Another civil war ensued, with Octavian victorious. Afterwards, he passed the laws giving him particular constitutional powers. From 27 BC, he would be known as Augustus, the first emperor of the Roman empire.

But it would be incorrect to suggest that the switch from republic to empire was an instant one. As historian Philip Matyszak notes, after Augustus’s ascension, “democracy didn’t really end” but continued in a lively form in most of the towns and cities of the empire.

“When we look at Pompeii and the graffiti there, we can see there were election campaigns going on, and apparently genuinely contested elections,” says Matyszak. “When he became emperor, Augustus was anxious to give the impression that the life of the Republic continued as before. It was only over the next century or so that this became more and more of a hollow charade. Augustus was keen not to tell everyone they were subordinate to him. That was what had got his adoptive father Casear killed, after all.

“Instead, he was known as the princeps, the first citizen. While he argued that he had no legal authority – although in reality he did, as he had command of several key provinces – Augustus was second to none in his personal authority.”

The Senate still functioned, but Augustus was definitely in control of government.

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From conquest to peace

The world ‘empire’ is suggestive of expansion and acquisition, of a land-grab that brought with it an increased population and the growth of the economy. However, there was relatively little expansion of Rome’s physical boundaries during the empire.

“The empire was largely in place by the time of Augustus,” says Matyszak. “The huge conquests of Gaul and the Middle East had been accomplished during the generation before that. When we look at imperial conquests, we’re looking at Dacia [a region that’s largely within the borders of Romania today] and at Britain. Egypt can be considered a republican acquisition because Augustus took it over before he became emperor. So there are few really major expansions during the empire.”

As the first ruler of the Julio-Claudian dynasty, by securing Rome’s borders, Augustus brought a sense of peace and prosperity after a period of political turmoil and upheaval. This prolonged time of stability became known as the Pax Romana. Not only did Augustus lay the metaphorical foundations of the empire, he also commissioned a substantial programme of building works, including the construction of the first Pantheon. He himself declared that he had “found Rome a city of clay but left it a city of marble”.

The founding of a dynasty

On his death in AD 14, Augustus was succeeded by his stepson Tiberius, who lacked the vision of his father. The remaining emperors of the Julio-Claudian dynasty were also pale shadows of the first emperor when it came to civic duty. Tiberius’s great-nephew Caligula succeeded him, but his four-year reign is remembered for his infamous predilection for sadism and cruelty. Next came Claudius, who was definitely an improvement on Caligula. He was a fine administrator, with an ambitious eye; the Roman conquest of Britain began during his reign.

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If Pax Romana described the relatively settled state of the wider empire across several generations, the political machinations in Rome itself were decidedly tempestuous. The Julio-Claudian dynasty ended with the suicide of Claudius’s successor, his great-nephew Nero, who was one of the empire’s most brutal leaders – a man who ordered the murders of both his mother and his first wife, and whom Romans popularly believed actually started the Great Fire of Rome in AD 64.

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A period of deeper unrest then ensued, with Rome descending into a series of power struggles; the year AD 69 would see no fewer than four men declaring themselves emperor. The subsequent Flavian dynasty – Vespasian, and his sons Titus and Domitian – then returned peace and stability.

Although his rule lasted just two years, Titus was a particularly effective emperor, one forced to show great leadership in the face of disaster and adversity, namely the eruption of Mount Vesuvius in AD 79, which buried Pompeii and Herculaneum under a rain of volcanic rock and ash, and a second major fire in Rome the following year.

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Further growth, expansion and prosperity came with the dawn of the Nerva-Antonine dynasty in 96 AD. Nerva, and the four emperors who followed him, presided over an extremely settled period. This was the high-water mark of the empire, with Trajan, Hadrian, Antoninius Pius and Marcus Aurelius truly consolidating Rome’s power. Now controlling the entire Mediterranean coastline, the empire covered a territory of nearly two million square miles.

Roman empire map: how large did the Roman empire become?

The Crisis of the Third Century and the Tetrarchy

The history of the Roman empire is an undulating one, with extended periods of stability counterbalanced by times of great chaos and disorder, often featuring emperors being assassinated before their allotted time.

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After the Nerva-Antonine dynasty came to a close with the demise of Marcus Aurelius’s successor Commodus (he was strangled in his bath), the Year of Five Emperors saw another multilateral struggle for power, resulting in the Severan dynasty from AD 193 onwards. This latest lineage saw expansion into Africa, as well as the extension of Roman citizenship to all free men across the empire – although this measure may have had less to do with noble intentions and more to do with raising Rome’s taxation income.

The empire was too unwieldy, too large, to be wholly governed by one central government from Rome

Political in-fighting – often resolved by assassination – dragged the empire into more chaos and into a period known as the Crisis of the Third Century, or The Imperial Crisis, which lasted from AD 235 to 284. This was a time of perpetual civil war as a procession of military leaders vied to become emperor. The empire effectively split into three, before being reunified by Aurelian in AD 274. But this unification lasted little more than a decade; the empire was too unwieldy, too large, to be wholly governed by one central government from Rome.

Enter Diocletian: Aurelian’s successor first appointed Maximian as his co-emperor in AD 286 and then, in AD 293, created the Tetrarchy, in which governance of the Roman empire (though not the empire itself) was split into East and West, each managed by a senior emperor (an augustus) and a junior emperor (a caesar).

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Principate becomes Dominate

Diocletian’s rule is significant in another way. His radicalism saw the term dominus (master) added to the emperor’s title. This was a major shift in how the emperor viewed himself, a clear stepping-away from the idea of the Principate. The remaining life of the Roman empire was now defined as the Dominate. “In the first half of the empire, the emperor is known as the first citizen,” explains Matyszak. “He’s increasingly seen as the person who sets the tone for the empire. Then, in the second half of the empire, the emperor becomes god of his domains. He is suddenly unquestionable. His word is law.”

The parts of the empire were often governed separately from one another, but not always. In AD 324, Constantine the Great defeated his co-emperor Maxentius to become sole ruler of both East and West. His rule was also significant for decreeing that religious tolerance be upheld towards Christianity. Indeed, the presence of Christianity has often been cited as a major contributory factor towards the ultimate fall of the Western Roman empire, at odds with the broad paganism that this half of the empire was largely living by.

Plus, the Western Roman empire was in military and economic disarray compared to its counterpart in the east. The latter would survive for a further thousand years. The western half, however, is usually given a date of death of AD 476, the year that the rule of the final emperor ended.

“The empire had been steadily falling under the control of barbarian warlords for many years,” says Matyszak. “The last Roman emperor was Romulus Augustulus, who was basically a figurehead. The state was being run by a German barbarian called Odoacer, who decided there was no need for an emperor of the Western Roman empire. So they exiled him. That tells you something of how diminished the role of Roman emperor was by that point. They didn’t even bother killing him.”

A slow decline, not a quick death

Despite the AD 476 date, there was no great fall of empire, no sudden, cataclysmic event that marked an absolute end-point.

“If you were to tell somebody in AD 476 that the Roman empire had just fallen, they would have looked at you as if you were mad,” says Matyszak. “People were still going to the voting booth to choose the public officials for the year. They were still going to the arena to watch the chariot races. For them, life just carried on as usual. The date of AD 476 was dreamed up by historians in the early modern era.”

For an empire that redefined the idea of civilisation and society, it was an ignominious end. “Rome didn’t so much fall, as gradually collapse and fade away,” says Matyszak.

Roman empire timline: key dates from Augustus to Romulus Augustulus

27 BC | Augustus, the great-nephew and heir of Julius Caesar, takes power, becoming Rome’s first emperor and ending the Roman Republic – which had existed for nearly five centuries.

AD 43 | The conquest of Britain begins. The province of Britannia would be part of the Empire for 367 years, but it took 30 years for the island to come under Roman rule – bar the far north.

AD 64 | A great fire destroys much of Rome. Emperor Nero blames the Christians for the disaster and has many of them killed.

AD 79 | Mount Vesuvius, a volcano near modern Naples, erupts and buries the cities of Pompeii and Herculaneum in ash.

AD 80 | Construction of the Colosseum is completed in Rome. This grand amphitheatre, the heart of entertainment in the empire’s capital, was the largest of its kind ever built and could hold 50,000 spectators.

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AD 117 | Under Emperor Trajan, Rome’s territory is at its largest – spanning from Iberia to Mesopotamia and from Britain to Egypt.

AD 165 | A plague is brought to Rome by soldiers returning from the Near East that kills an estimated five million people. Historians believe the cause was smallpox.

AD 193 | After the murder of Emperor Commodus, five men claim the imperial title. Septimius Severus would be the final victor, founding the Severan dynasty.

AD 235 | The Crisis of the Third Century begins – a period where Rome suffers invasions, plague, an economic recession and many short-lived emperors, all fighting to rule.

AD 293 | Diocletian puts an end to Rome’s crisis and divides governance of the empire into four. Known as the Tetrarchy, four men ruled the empire, two in the West and two in the East.

AD 324 | Constantine the Great, the first emperor to convert to Christianity, reunites the empire again and becomes the sole emperor.

AD 380 | Emperors Gratian, Valentinian III and Theodosius I issue the Edict of Thessalonica, declaring Christianity the only official religion in the Roman empire and ending state support for polytheism.

AD 395 | Theodosius I dies, having become sole emperor in AD 392. At his passing, governance of the West and the East is split between his two sons, never to be united again.

AD 455 | The Vandals, a Germanic tribal people, sack Rome. The once great city is systematically plundered.

AD 476| The final emperor in the west, Romulus Augustulus, is ousted during a revolt of Germanic ‘barbarians’. This is considered by some as the fall of the Roman empire.

Dr Philip Matyszak is an expert ancient history, with a focus on ancient Rome; Nige Tassell is a freelance journalist specialising in history

This content first appeared in the September 2020 issue of BBC History Revealed