In his speech of praise to Rome (quoted below, p. 510) Aelius Aristides portrayed the inhabitants of the Roman empire as safe inside well-guarded and impenetrable frontiers. This was misleading. The concept of frontier took some time to mature. During the republic the vision of world conquest persisted and the idea that the empire should set limits to its expansion was never articulated. It was during the reign of Augustus that the concept first took root in face of the difficulties and disasters experienced in conquering the German tribes. Tiberius consolidated the policy and from then on the frontiers of the empire began to stabilize. Britain and Dacia were the only new areas conquered and held for any length of time after the reign of Augustus. Trajan may have dreamed of emulating Alexander in achieving new conquests to the east but his successor Hadrian recognized the impossibility of maintaining control of so vast an empire if its armies were also set on further conquests. So he called a halt and began to consolidate the frontiers, most famously in the wall he built across northern Britain.
By the time the empire had reached its fullest extent, in the second century ad, there were in fact thousands of kilometres of boundaries to defend. From the mouth of the Rhine to that of the Danube is 2,000 kilometres. The borders of the north African provinces ran for a total of twice that. The eastern frontier still had a number of client kingdoms and no fortified frontier was necessary until these had been absorbed, as Cappadocia, Nabataea, Commagene were in the first century ad and Palmyra and Osrhoene in the second. (Armenia remained as a buffer state between Rome and Parthia.) The shortest marching route between the Black Sea and the Red Sea was then 3,000 kilometres by road.
Sometimes the frontiers were natural boundaries, such as rivers or mountains. Others ran over open desert or through woodland. It was obvious that these could not all be protected by military force, nor was there any particular wish to close the empire off from the world beyond. Rome needed its luxuries, amber and fur from the Baltic, silks from China, spices from elsewhere in the east, and gold from deep within Africa. Barry Cunliffe argues that Rome’s greatest need was for slaves—an estimated 140,000 were required annually to maintain the supply of the empire— and so contacts had to be sustained outside the empire. In effect, except in a few defined trouble spots, the frontiers of the empire were permeable. Even Hadrian’s Wall, the most sophisticated and complete barrier between the empire and the outside world, was designed to be crossed by traders entering or leaving under Roman supervision and its main purpose may have been to control this communication more effectively. (See the lively account in Alistair Moffat, The Wall: Rome’s Greatest Frontier, Edinburgh, 2008. The English Heritage Guide, Hadrian’s Wall, with reconstructions, maps, etc., is by David Breeze, London, 2006.)
This does not mean there were not fortifications along the borders of the empire. Hadrian’s Wall in Britain is, as just noted, the best-known example, but in Germany there was the limes, originally a military road overseen by watchtowers. Under Hadrian continuous palisades were constructed to strengthen it. In Africa ditches and watchtowers were built to protect the prosperous grain - and olive-growing areas from raids by nomadic tribes. (These tribes were never a serious security threat and no more than 45,000 troops (legionaries and auxiliaries) were allocated for the entire 4,000-kilometre frontier.) In the east there were no formal lines of defence but roads were built running back from the frontier with Parthia so that troops could be rushed to the front if needed.
In reality the defence of the empire depended not on walls but on a mixture of diplomacy and the ultimate threat of armed force. The native tribes could be bought off by gifts of money or luxury goods, or offered special protection. The sons of leaders could be brought into the empire, ‘civilized’ in the emperor’s household, and then returned home, hopefully as lifelong friends of the Roman people. Disputes between tribes could be fostered so that they did not unite against the empire. Tacitus, as usual, put it shrewdly: ‘May the tribes ever retain, if not love for us, at least hatred for each other.’ There were always dangers along the northern border simply due to the shifting allegiances and relationships of the various tribes, and when pressures built up among them from the end of the second century diplomacy was not to be enough. It was easier, in theory, to deal with Parthia, a centralized state with only one ruler with whom to negotiate. (See Philip Parker, The Empire Stops Here: A Journey along the Frontiers of the Roman World, London, 2009, for a personal but erudite exploration of the Roman frontiers.)
World History
