I. The Roman Frontier Defence
The Roman Empire was the result of a process of conquest which began some 400 years before the Christian era and continued into the second century A.D. Yet it might be said of this empire, as of the British Empire, that it was acquired “in a fit of absent-mindlessness.” Though the Romans from time to time committed breaches of international morality and of the rules of war as then understood, it cannot be said that they pursued a systematically aggressive policy. Their historians could plausibly claim that Roman wars, as a rule, had been fought in defence of the city or of the common peace of the Mediterranean; and modern writers on the same subject at least admit that the Romans groped their way to world-power rather than hurled themselves upon it. Moreover, about the beginning of the Christian era Roman foreign policy underwent a remarkable change. Until then it had been essentially opportunistic, henceforward it was based on a clear and consistent principle. Rome deliberately broke off her career of conquest and contented herself with holding her past gains. This new rule originated with the Emperor Augustus, and was perhaps his most notable contribution to Roman statesmanship. In framing it Augustus was scarcely influenced by cosmopolitan or humanitarian sentiment. But he realised that the Roman Empire had reached that important turning-point at which warfare would cease to pay, for it had already absorbed those peoples who owned large stocks of transferable wealth and could thus be made to pay for the costs of their conquests. As a sound financier he set his face against a ruinous policy of further expansion. Besides, after the wars which occupied the earlier part of his reign and extended Roman rule to the Danube, he discovered that the existing frontiers of the Roman Empire were as strong as Nature could provide, and enclosed an area of countries grouped round the Mediterranean Sea which was in itself a natural geographic unit. Augustus therefore inserted emperors to eschew testament an admonition to future emperors for the most part were content to obey.
it is true that the rule laid down by Augustus suffered some exceptions, and in Western Europe a few more wars of expansion were waged. Augustus himself for some years entertained the idea of conquering himself for some years entertained the idea of conquering Germany as far as the Elbe. In support of this policy it could be argued that a line running from the mouth of the Elbe to Bohemia, and from that point to the bend of the Danube below Vienna, would be shorter than the two converging lines of the Rhine and Upper Danube, but a defeat which his forces sustained in the jungles of Northern Germany (A.D. 9), though nothing more than an incident in a generally successful war, sufficed to spoil the emperor’s stomach fighting, and his scheme of conquest in Germany was incontinently abandoned. In the second century A.D., an extension of the Roman frontier from the Lower Danube to the Carpathians, which was undertaken by the Emperor Trajan (A.D. 105), raised the question of a corresponding advance to the line of the Elbe. But no further move was prepared until the last days of Marcus Aurelius (A.D. 180), and his death made an end for the time being to a policy of fresh annexations. In A.d. 234-235 a general named Maximinus, as a result of a successful retaliatory raid across the Rhine, resumed the plan of pushing the frontier forward to the Elbe, but his project, like that of Marcus Aurelius, died with him shortly after. In connection with the scheme of frontier fortifications which we shall presently consider, various Roman emperors of the first three centuries slightly encroached on the farther banks of the Rhine and Upper Rhine, but in so doing they were strengthening rather than abandoning the existing boundary line.
While the German frontier was thus being stabilised, a notable extension of Roman territory was made across the Channel. In A.d. 43 the Emperor Claudius revived the scheme of conquering Britain, which Julius Caesar had formed but Augustus renounced on the ground of its costliness. The reasons for this reversal of policy, and the story of the actual conquest, will be found in the companion volume on Roman Britain. Suffice it here to say that by A.D. 84 the Romans had advanced to the foot of the Grampians, that during the greater part of the next century they occupied the Scottish lowlands, and that in A.D. 211 they finally chose the line of the Tyne and the Solway for their boundary.
In selecting their frontiers the early Roman emperors were guided by a geographical principle. In the words of Augustus, “the empire should be kept within the bounds of ocean, desert, and distant rivers.” On their sea front the only countries which the Romans could possibly regard as a menace were the British Isles. Of these, Britain was soon made safe by annexation; Ireland was left severely alone, and repaid this neglect by prolonged non-interference in Roman affairs. On the Continent the advantage of the river boundary lay not so much in the difficulty which invaders would have in crossing the water, as in the case with which the defenders could patrol it laterally with the help of their ships and their frontier roads. On the farther banks of the Rhine and the Danube the Romans contented themselves at first with maintaining a glacis of uninhabited land and a few outposts across the Middle Rhine in the Taunus district, among them Wiesbaden. But Vespasian (c. A.D. 75) cut off the re-entrant angle near Basel by annexing the triangle between the Rhine , Danube, and Upper Neckar. Shortly afterwards Domitian traced out a new advance line along the Taunus watershed and the Main valley, and so to the Upper Danube, and defended it with a double series of forts, of which the now reconstructed Saalburg (in the Taunus) may serve as a specimen. About A.D. 125 Hadrian further strengthened this line, and marked it off as a definitive frontier by means of a continuous palisade. Nevertheless, hid successor, Antoninus Pius, threw out a row of forts to the east of the Neckar and the north of the Upper Danube, and soon after A.D. 200 these were connected with a continuous stone wall (on the Danube sector) or a palisade and ditch (along the Neckar sector). But the forts were mere outposts, and the barrier chiefly served as a customs frontier. The main line of defence always lay along the rivers, and all the larger Roman camps were situated on the left bank of the Rhine. The headquarters were at Mainz and at Birten (Castra Vetera, near Cleve); other large stations were at Neuss, Bonn, Strasbourg, and Windisch (on the Aar). The camps served as bases for field operations rather than as obstacles to invasion, for the Roman army of the first two centuries A.D. was essentially a moble force which put its trust in its arms and legs, not in bricks and mortar.
The Roman army in the western half of the empire had a regular establishment of about 120.000 men. Originally two-thirds of this force were stationed along the Rhine and Upper Danube; after the conquest of Britain, which received a standing garrison of 30.000 men, the army on the German sector was reduced to 60.000. Outside the frontier zone the number of regular troops was extraordinarily small. In Spain Augustus had left a force of 30.000 men, whose main object was to watch the still unruly tribes of the north-west. By A.D. 100 it was found safe to reduce this garrison to 10.000. In Italy the imperial guards (“praetoriae cohortes”), 10.000 strong, served to defend the emperor rather than the empire. In the interior of Gaul the only standing force was a police battalion at Lyon.
In the days of the great conquests the Roman army had been recruited exclusively from Italy; but from the time of Augustus it was found necessary to draw upon the outlying peoples as well, and by A.d. 100 these were supplying almost the whole of the forces. The western populations in particular – Gauls, Spaniards, and Britons – enlisted readily in Rome’s service. At first the various units changed their quarters not infrequently, and some emperors made a point of removing recruits from their native districts; in Britain the garrison contained details from Spain, the Danube lands, and even Syria. Eventually, however, the greater convenience of local recruitment asserted itself, and by the third century the Roman army was changing into a territorial force.
Almost to the end of Roman history the flower of the army consisted of infantrymen. Of these one half were enrolled in the “legions” – i.e., permanent regiments of 5.000 to 6.000 men equipped with a heavy javelin, a stabbing sword, a large semi-cylindrical shield, and a moderate amount of body armour. The so-called “auxiliaries,” who were largely drawn from the regions on the outskirts of the empire, served in smaller units and usually carried a lighter armament. The cavalry, which had been strangely neglected under the Republic, received more attention in the period of the emperors, but in the first two centuries A.D., its functions remained subordinate. The Roman artillery pieces, which were discharged by the sudden release of a twisted rope, could propel heavy stones or armour-piercing arrows over an effective range of about a quarter-mile. But they had too slow a rate of fire to be of service in the field.
The Roman navy consisted wholly of cruisers and transports. Two of its squadrons, stationed at Misenum (near Naples) and at Forum Iulii (Frejus, near Toulon), policed the Western Mediterranean; another wish its headquarters at Boulogne protected the North Atlantic coasts against pirates. The flotilla on the Rhine was chiefly used for transport.
The strength of the Roman army lay above all in its rigorous training. Originally a conscript militia, it had in the last century B.C., been converted into a professional force. From the time of Augustus the term of enlistment was fixed at sixteen, twenty, or even twenty-five years. The same emperor made careful provision for the regular payment of the troops, which was on a fairly liberal scale, and for their pensions (in money or land). Hence, although the right of conscription was never renounced, in normal times voluntary enlistment sufficed to fill the gaps. Under the earlier Roman emperors the officer corps was not yet completely professionalised, but tended more and more to become so. Its backbone was furnished by the centurions, who doubled the part of drill sergeant and of company commander. These were mostly men of proved merit and great experience, who had gained promotion from the rank and file. Between battles the Roman soldiers were inured to remorseless labour in marching and digging; in battle they were adepts at combined operations and could equally well skirmish or employ shock tactics. From time to time, of course, individual units were caught napping by enemy raiders, and during occasional spells of civil war the frontiers were seriously endangered. But on the whole the army of the earlier Roman emperors was a remarkably efficient instrument of defence. The great development which Europe underwent at the beginning of the Christian era was in no small measure due to the almost unbroken security which the Roman soldiers maintained on its behalf.