The momentum of expansion dwindled and was redesigned into the momentum of defense. The Roman military had formerly focused on conquering new land and transforming such land into integrated provinces of the Empire. Now, the military was more interested in ensuring that competing powers - like the Germanic tribes, the Scots, and the Irish - did not expand into Roman land.
In addition to stationing garrisons along the border, the Romans, ever the good engineers, undertook another amazing building project: a series of walls which would eventually stretch for hundreds of miles along selected segments of the empire’s borders.
The empire sprawled across Africa, Asia, and Europe, and included islands like Great Britain. The borders totaled thousands of miles. Historian Andrew Curry describes how
A stunning network of walls, rivers, desert forts, and mountain watchtowers marks Rome’s limits. At its peak in the second century A.D., the empire sent soldiers to patrol a front that stretched from the Irish Sea to the Black Sea as well as across North Africa.
As a percentage of the total imperial border, the walls were a small fraction, built where strategists figured they were most needed. The engineering precision is impressive. In one case, a 31-mile stretch of wall is almost perfectly straight, deviating merely 36 inches. The design of the wall is precise and crisply geometrical. The exact shape of the wall varies: Hadrian’s Wall between England and Scotland is a different structure than the Limes wall in Germany.
Why did the Romans build the walls? To protect a regime besieged by barbarians, or simply to establish the physical edge of the empire?
The walls were only a small part of the border system. More often, there were watchtowers spaced at intervals. How porous were these borders? Certainly, local Germanic and Celtic tribesmen in central Europe were used to trading with each other, and if a Roman border ran between two small settlements, that would have meant little to them, and little to the Roman military men stationed there. The borders were most likely surveilled for the purposes of watching for major military movements.
But such imperial thinking was foreign to the original Roman Republic, a governmental structure dating from around 509 B.C., and designed to administering a city-state and a few agricultural lands surrounding it. The Republic’s success in expanding would also be its downfall.
From around 500 B.C., Rome expanded continually for six centuries, transforming itself from a small Italian city-state in a rough neighborhood into the largest empire Europe would ever know.
The Republic was not capable of effectively governing this large territory. The Empire replaced it.
The emperor Trajan was an eager heir to this tradition of aggression. Between 101 and 117, he fought wars of conquest in present-day Romania, Armenia, Iran, and Iraq, and he brutally suppressed Jewish revolts. Roman coins commemorated his triumphs and conquests.
Trajan left a gigantic empire to his successor. Had Trajan lived longer, he might have learned that the empire was perhaps too big to be thoroughly organized and successfully defended.
The military requirements - defending borders in England, across central Europe, into southwestern Asia, and across northern Africa - were enormous. Too big, in fact. The regular Roman army was supplemented at first by domestic mercenaries, then by foreign mercenaries. But this help was often actually another problem.
When he died in 117, his territory stretched from the Persian Gulf to Scotland. He bequeathed the empire to his adopted son — a 41-year-old Spanish senator, self-styled poet, and amateur architect named Publius Aelius Hadrianus. Faced with more territory than Rome could afford to control and under pressure from politicians and generals to follow in the footsteps of his adoptive father, the newly minted emperor — better known as Hadrian — blinked. “The first decision he made was to abandon the new provinces and cut his losses,” says biographer Anthony Birley. “Hadrian was wise to realize his predecessor had bitten off more than he could chew.”
The Romans had ventured northeast of the line on which the Limes wall would eventually be built. The earliest walls had actually been built as far back as the reign of Octavian-Augustus, who suffered a humiliating defeat in 9 A.D. at the hands of the Germanic tribes.
Under Hadrian's rule, they would pull back a few miles to more defensible positions.
But under Hadrian and his successor, the Limes boundary line, roughly the southwest border of Germany, would reach its full structural development of walls, watchtowers, and forts.
The new emperor’s policies ran up against an army accustomed to attacking and fighting on open ground. Worse, they cut at the core of Rome’s self-image. How could an empire destined to rule the world accept that some territory was out of reach?
As it turned out, the Roman Empire had not only stopped its expansion, but it was preparing to shrink. The Germanic tribes in Europe, and the Scots in Great Britain, became bolder and more familiar with Roman military practices. Knowledge of the Romans allowed the tribes to strategize ways to outmaneuver and outfight the Romans.
The Limes marked the highpoint of the empire, but at the same time marked the beginning of the end.
