What was the Roman Empire? Its rise and fall, explained

The Roman Empire was the state ruled by emperors that grew out of the Roman Republic in 27 BC, when Augustus became its first emperor. At its height, around AD 117, it ran from Britain to Mesopotamia, ringed the whole Mediterranean, and held about 55 to 60 million people by the traditional estimate, perhaps more. Its western half broke apart in the 5th century, traditionally dated to AD 476. The eastern half, ruled from Constantinople, lasted until 1453.

In short

• Rome became an empire not by conquest but by a change of government: the republic's conquests were already vast when Augustus took sole power.
• It held together through its army, its roads, its law and, slowly, by making its subjects Roman citizens.
• The West fell through many causes acting together; the richer, better-defended East survived for another thousand years.

How did the Roman Republic become an empire?

Rome had been a republic since, by tradition, 509 BC, run by elected magistrates and a Senate of aristocrats. By the 1st century BC its conquests covered most of the Mediterranean, but the republic's institutions, built for a city-state, could not control the generals who won them. Ambitious commanders with loyal armies fought a series of civil wars.

Julius Caesar won one of them and was named dictator for life, then was murdered by senators in 44 BC. His adopted heir, Octavian, defeated his last rival, Mark Antony, and Antony's ally Cleopatra at the Battle of Actium in 31 BC. In 27 BC Octavian formally handed power back to the Senate, and the Senate gave him the name Augustus. In his own account, the Res Gestae, he wrote that "I transferred the republic from my own control to the will of the senate and the Roman people." In practice he kept command of the army and the key provinces, and every emperor after him ruled behind the forms of a republic.

	Republic (509–27 BC)	Empire (27 BC onward)
Who held power	Two consuls elected each year, the Senate, popular assemblies	One emperor, with the Senate as an honored advisory body
Who commanded the army	Elected magistrates, for limited terms	The emperor, permanently
How power passed	Annual elections	Inheritance or adoption in theory; often murder or civil war in practice
Size	Grew from one city to the Mediterranean	Stabilized at its greatest extent around AD 117

How did Rome run such a vast empire?

Through a handful of institutions that reached almost everywhere. The army, a professional force of about 25 to 30 legions of roughly 5,000 men each plus as many auxiliary troops, guarded frontiers along the Rhine, the Danube and the desert. A network of paved roads, built first for the legions, carried officials, trade and news. Cities, each with its own council, did most of the day-to-day governing and tax collecting, so the empire needed only a small central bureaucracy.

Roman law gave the empire a shared framework for property, contracts and inheritance, and its later codification under the eastern emperor Justinian in the 6th century became the basis of the civil law used across much of continental Europe and Latin America. Citizenship spread slowly, as a reward and a tool of loyalty, until in AD 212 the emperor Caracalla extended it to almost all free inhabitants. Latin dominated in the West and Greek in the East, and local gods, languages and customs largely carried on.

The first two centuries, from Augustus to the death of Marcus Aurelius in 180, are often called the Pax Romana, the Roman peace. It was peace inside the frontiers, kept by war along them, and it rested on the labor of millions of enslaved people.

What went wrong in the third and fourth centuries?

After 235 the empire nearly broke apart. In the Crisis of the Third Century, about 50 years of civil wars, invasions, plague and debased coinage, more than 20 emperors rose and fell, most of them killed. Diocletian, emperor from 284, restored order by dividing rule among four emperors, enlarging the army and the bureaucracy, and raising taxes.

Constantine, who won power in 312, made Christianity a favored religion and in 330 founded a new capital at Byzantium, renamed Constantinople. In 380 Christianity became the official religion. After the emperor Theodosius I died in 395, the empire was permanently divided between his two sons, one ruling the West and one the East.

Why did the Western Roman Empire fall?

There is no agreed single answer. The German historian Alexander Demandt once collected 210 explanations that people have offered. The chain of events is clear enough. In 376 Goths fleeing the Huns crossed the Danube, and in 378 they destroyed an eastern Roman army at Adrianople, killing the emperor Valens. Around 406 other groups crossed the Rhine into Gaul. The Visigoths sacked Rome in 410, the Vandals took the rich province of Africa in 439 and sacked Rome again in 455. The western army came to be made up largely of barbarian troops under their own leaders. In 476 one of those commanders, Odoacer, deposed the last western emperor, a boy called Romulus Augustulus, and did not bother to name another.

Still argued

Historians weigh the causes very differently, and most now see several working together.

Explanation	The argument	Associated with
Internal decay	Political instability, heavy taxes and a weakened civic spirit, with Edward Gibbon also blaming Christianity	Edward Gibbon, from 1776
Outside pressure	Invasions and migrations, set off by the Huns, overwhelmed a still-functioning state	Peter Heather
Violent collapse	The end brought a real fall in living standards, trade and literacy, not just a change of rulers	Bryan Ward-Perkins
Transformation	Roman culture, law and the Church carried on under new kings; "fall" overstates the break	Peter Brown and the "late antiquity" school
Climate and disease	Epidemics, such as the Antonine Plague, and a colder climate later weakened the empire's base	Kyle Harper

The summary of the debate on Wikipedia and the World History Encyclopedia both conclude that no single cause explains it.

Why did the East survive for a thousand more years?

The East had richer, more populous and more urban provinces, such as Egypt, Syria and Anatolia, which meant more taxes and more soldiers. Its main frontier was shorter, and invaders who reached it could often be paid to go west instead. Constantinople, protected by the sea and by massive land walls built in the 5th century, could not be taken by the armies of the time.

Its people called themselves Romans for its whole history; "Byzantine Empire" is a label historians gave it later. In the 6th century Justinian won back Italy and North Africa. Then the plague of 541, caused by the same bacterium as the Black Death, and the Arab conquests of the 7th century weakened it and cost it much of the East. It survived as a smaller Greek-speaking state until 29 May 1453, when the Ottoman Empire took Constantinople.

What did Rome leave behind?

Romance languages, from Spanish to Romanian, descend from Latin. Roman law shapes legal systems on several continents. The Latin alphabet, the calendar of months, the shape of the Catholic Church, the layouts of cities from London to Cologne, and the words "senate" and "republic" all come down from it. Even the Romans' founding myth, that they descended from the Trojan prince Aeneas, kept the story of Troy alive. The question of why it fell is one of history's longest-running arguments, and each age has answered it partly with its own worries.

Check yourself

• Without looking back: what changed in 27 BC, and what stayed the same in form?
• Name three institutions that held the empire together, and say how each worked.
• Why did the western half fall in the 5th century while the eastern half survived?

Sources

• Roman Empire — World History Encyclopedia, Joshua J. Mark, 2018
• Fall of the Western Roman Empire — World History Encyclopedia, Donald L. Wasson, 2018
• Res Gestae Divi Augusti, translated by Frederick W. Shipley — LacusCurtius, University of Chicago, 1924 translation
• Fall of the Western Roman Empire — Wikipedia
• Roman Empire — Wikipedia