Why Did the Roman Empire Stop Expanding?

At its peak, the Roman Empire extended from Spain to Syria, from Scotland to Sudan – covering

something like five million square kilometers (almost two million square miles).

It encompassed the entire Mediterranean Basin, and perhaps a quarter of the human race.

Even more impressive than the sheer scale of the empire was its longevity.

For half a millennium, the Roman Empire was – with the possible exception of Han Dynasty

China – the largest, richest, and most powerful state on the planet.

The Roman frontiers were remarkably stable.

A map of the empire during the reign of Augustus looks very similar to a map of the empire

in the reign of Theodosius I, four centuries later.

This video will consider why the empire remained the same size for so long – why, in other

words, after a long period of rapid growth, the Romans seemed to reach a series of tipping

points, beyond which they could not or would not extend their rule.

The political dynamics of the Roman Republic had encouraged expansion.

Ambitious members of the elite, knowing that military success was the shortest way to political

prestige, started wars and annexed territory for Rome’s glory and their own.

They almost always found opportunities to do so, since Rome expanded into a polycentric

world seething with conflicts and rivalries.

The hectic final decades of the Republic witnessed the vast conquests of Pompey in the east and

Caesar in the west.

But it was Augustus, the first emperor, who did most to give the Roman Empire its familiar

shape.

After he defeated Antony and Cleopatra and annexed Egypt, his lieutenants embarked on

a series of wide-ranging campaigns.

Northern Spain, which had defied the legions for almost two centuries, was finally added

to the empire.

The German frontier was pushed beyond the Rhine.

The Alps were definitively conquered, as was the whole Balkan peninsula as far as the Danube.

The frontiers would shift over the next century and a half.

Germany would be abandoned after the disaster in the Tuetoburg Forest.

Britain would be conquered under Claudius, and Dacia under Trajan.

Client kingdoms would be absorbed on all sides.

But in general, the empire established by Augustus would change little for four centuries.

This might seem unsurprising.

Much of the Republic’s expansion, after all, had been driven by the ambitions of competing

politicians; with those politicians and their rivalries gone, it might be assumed that the

engine of expansion had been switched off.

Augustus himself reportedly advised his successors not to expand the empire further.

But emperors still needed military glory.

Claudius invaded Britain to validate a shaky political position, and Hadrian was subjected