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 5 million square kilometers, almost 2 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 Teutoburg 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 to fierce criticism after he relinquished Trajan's Mesopotamian conquests to the Parthians. Since the political, or at least propagandistic, benefits of conquest were still so high, why did the frontiers become so static? The Roman frontiers, it should be noted here, were different from modern national frontiers. A modern frontier is a line of division, often mapped onto a river or some other geographic feature. The Roman conception was more expansive, a zone of control, as opposed to a fixed boundary. Roman poets liked to claim that the power of the emperors was boundless, and that the peoples beyond the charmed circle of the provinces were independent by sufferance. Although this was bombast, governors and commanders along the empire's edges were expected to project authority well beyond the bounds of their provinces. That said, those bounds existed and seldom moved. In some places, the Roman frontiers reached clear ecological boundaries. This was most evident in North Africa, whose provinces terminated at the Sahara Desert. The Arabian and Syrian deserts were almost equally forbidding. These were not impassable barriers. Nomadic raiders from the desert edge would cause headaches for centuries of provincial governors. But with the exception of Augustus' abortive attempt to conquer Yemen, the Romans never tried to extend their control deep into a desert region. The Armenian highlands and Atlas Mountains were the only places where the Roman frontier coincided with a mountain range. Elsewhere, in the Alps, in the Carpathians, and in the Balkans, the Romans absorbed mountainous The frontiers also disregarded climatic boundaries. Most of the Mediterranean coast, including almost the entire Italian peninsula, experiences hot, dry summers and relatively cool, wet winters. It was only here that olive trees, an important source of food and fuel throughout the Roman period, could grow. Grapevines, more resistant to winter frost, could be cultivated through much of temperate Europe. But perhaps a third of the empire's population lived beyond the limits of grape and olive cultivation. Likewise, although bogs and dense forests made farming difficult in parts of central and northern Europe, Roman frontiers stopped well short of the northern limits of efficient wheat and barley production. Parts of the frontier seem, however, to have reached what might be called a demographic boundary. At many points north and east of the Rhine and Danube, pre-Roman settlements were widely dispersed and towns non-existent. In terms of tax revenue, there was little to be gained from such places. This doesn't mean, of course, that the frontier was destined to stop where settlement thinned and tax receipts dwindled. Augustus pushed the German frontier to the Elbe and probably would have driven further east, had he not been checked at the Teutoburg Forest. Given another few years, Agricola could have conquered all of Scotland, and Marcus Aurelius might well have annexed much of central Europe. But it's hard to escape the feeling that the Romans never pressed farther north because they simply ran out of good territories to conquer. In the east, the Roman frontier bisected northern Mesopotamia, a region that was naturally unified both economically and culturally. But, with only brief exceptions, the Romans were content with only part of this lucrative and fertile area, leaving the rest to the Parthian, and later Persian, empires. Trajan briefly pushed the eastern frontier down to the Persian Gulf, and Caesar may have considered doing something similar. But most emperors seem to have regarded further conquest in this direction as beyond the bounds of political viability. There was glory to be won, but not cheaply. I recently had a conversation with Adrian Goldsworthy, whose forthcoming book investigates the 700-year rivalry between Rome and Persia. So, for a detailed look at Rome's most contested frontier, and deeper consideration of this video's basic question, I encourage you to follow the link in the description and check that video out next. If you're interested in more Tolandstone content, including my podcast, check out my channel, Tolandstone Footnotes. I also have a channel called Scenic Routes to the Past, which is dedicated to historically themed travel. You'll find both channels linked in the description. Please consider joining other viewers in supporting Tolandstone on Patreon. You might also enjoy my book, Naked Statues, Back Gladiators, and War Elephants. Thanks for watching.