There were, however, cases, particularly in warfare against barbarians, where the enemy would not meet the imperial forces in the open field, but took up a defensive position on ground where legionaries could not be employed with success. In these circumstances the auxilia formed the first line and began the attack, and only if they were driven back and pursued by the enemy did the legions come into action. The battle of Mons Graupius, in 84, was conducted on these lines,[321] and similar tactics seem often to have been employed by Trajan in Dacia.[322] In general, however, the auxilia play a very secondary rôle; we do not hear either of the cavalry being used to strike the decisive blow after the manner of Alexander,[323] or of any such combination of archers and heavy infantry as we find in mediaeval warfare.
The subject, however, is still obscure, and it is more satisfactory to turn to the part played by the auxilia in frontier defence, concerning which the archaeological research of the past twenty years has established more certain conclusions. In the frontier policy of the first two centuries we can trace two opposing tendencies at work, each of which is reflected in the disposition of the troops and the duties required of them. At the death of Augustus the Empire had as yet reached hardly any of its natural boundaries, although by means of the system of client kingdoms and ‘protected’ tribes it was asserting its claims and intentions in much the same fashion as the powers of modern Europe are doing in Africa to-day. The first century therefore witnessed on almost every frontier a period of expansion of greater or less duration, in which the sphere of direct administration was gradually pushed forward until some physical or political obstacle was reached which necessitated either a halt or a forward policy on a much larger scale. Throughout this period military operations were always imminent; in Britain, for example, between 50 and 85, the garrison marched out almost every spring, either for a campaign or a military demonstration. In winter, therefore, or in times of peace, the frontier armies were so disposed as to be able to take the offensive at a few days’ notice. The legions often lay in pairs, while many of the auxiliary regiments, instead of being scattered over a wide area, as was the case later, were concentrated at a few strategic points. The extent to which this system was adopted varied, of course, with local conditions, and a few regiments always occupied more isolated positions, but as a whole the auxilia of a province were far more easily mobilized than later when each regiment had its own castellum. On the Rhine frontier Haltern and Hofheim furnish examples of these large hiberna, dating from the beginning and the middle of the first century respectively,[324] and we find the same system continued for the defence of the Taunus district annexed by Domitian.[325] There is, indeed, here a chain of forts on the frontier, but they are of small size, with an average area of only 1½ acres. The bulk of the auxilia lay some way behind the frontier, in forts which held some two or three regiments apiece.[326] In Britain we have traces of a similar system at the same date. The ‘Agricolan’ fort at Barr Hill is a frontier post which would require some two centuries at most for its defence, while the early fort at Newstead, which was probably occupied from about 80 to 100 or later, could accommodate at least 1500 men.[327] The essentially temporary nature of such hiberna is emphasized by the character of their defences, which usually consist simply of an earth wall or palisade, little more elaborate in construction than the vallum which an army in the field was expected to throw up round its camp after a day’s march.
In provinces whence archaeological evidence is not forthcoming, inscriptions indicate the same system. From Spain, for instance, we have an early inscription referring to an officer who held command over four cohorts,[328] and a similar brigade of three cohorts appears at Syene in Egypt in the reign of Trajan.[329] On the Danube frontier von Domaszewski has concluded from the epigraphical evidence that Aquincum and Arrabona each held two alae in the first century.[330]
This period of expansion may be considered to end with Trajan’s annexation of Dacia and his failure a few years later to execute a similar forward move on the Eastern frontier. With the accession of Hadrian a new policy begins, which advertised by the elaborate character of its defensive measures that the imperial government was firmly determined to renounce all further schemes of aggression, a determination which was adhered to until the power of decision lay no longer in Roman hands. The outward signs of this new spirit were the abandonment of the old hiberna, and the removal of their garrisons to stone forts of a new type, each arranged to hold no more than a single unit, which were placed at more or less regular intervals along the frontier instead of behind it.[331] The auxilia, that is, were transformed from a potential field army into a frontier police.
This policy of passive defence depended, of course, for its success upon the extent to which the frontier could be made defensible. Fortunately by this date it lay for the greater part of its length along positions of great natural strength. The Rhine, Danube, and Euphrates, when guarded by a continuous line of forts and watch-towers,[332] and patrolled by flotillas of guard-boats, formed a serious military obstacle to a raiding force, an obstacle even more dangerous to its retreat than its advance. The desert frontiers of Africa and Arabia were more easily defended, since the routes by which a hostile force could advance were limited in number and the defence could concentrate upon them, and the same of course holds true of the southern frontier of Egypt.
There were, however, districts where such natural obstacles did not exist, as in the case of the trans-Rhenane territory, which was divided between Germania Superior and Raetia, and the northern frontier of Britain, and here Hadrian had recourse to the expedient of erecting artificial barriers, which he hoped would serve the same purpose.[333] In the former case the frontier was defended by a palisade and ditch, which were later supplemented by an earth mound in the German section and replaced by a stone wall in Raetia.[334] On the British frontier, between the Tyne and Solway, the existing remains are those of a stone wall, although there are also traces of a wall of turf, which may have been an earlier work.[335] A turf wall also defended the more advanced line between the Firths of Clyde and Forth, which was occupied between 140 and 180.[336] The southern line in Britain in its most perfect form was guarded by a stone wall seventy-three miles long. This wall was between six and nine feet thick, and probably stood originally about twelve feet high. In front of it, except where the precipitous nature of the ground rendered such an additional defence unnecessary, ran a wide ∨-shaped ditch. At intervals of about every Roman mile stood a stone block-house, and between every block-house two towers. The mile-castles contained barrack accommodation for about fifty men, and reveal abundant traces of continuous occupation.[337] The garrison of about eleven thousand men lay in stone forts of the ‘cohort’ size, the majority of which are actually on the line of the wall, although a few, which probably belonged originally to an earlier system of defence, are a short distance behind it. The average interval between the forts is some six miles, so that it was easily possible for each regiment to man the adjacent towers and mile-castles and retain a considerable force at head-quarters. In addition to the troops actually stationed on the line of the wall, there were other regiments in outpost forts to the north and in the forts which guarded the three roads leading south to the legionary fortresses of Chester and York. The ends of the line were also guarded against flank attacks from the sea by forts at South Shields and on the Cumberland coast. If, then, we include all troops within three days’ march of the wall, the total force available for its defence probably exceeded twenty thousand men. Taking also into our calculations the natural strength of the position, we may safely say that this was the strongest and best guarded of all the frontier barriers.
The trans-Rhenane frontier, which extends for over three hundred miles from Rheinbrohl on the Rhine to Eining on the Danube, was defended by the same methods, although in certain sections the forts were more widely separated and the garrison was proportionately weaker. There were also fewer troops within call immediately behind the frontier line. Here also, between the cohort forts, stone ‘Zwischenkastelle’ and ‘Wachttürme’ furnished additional safeguards.[338]
This whole system of frontier defence has been much criticized, and the limitations and possibilities of these artificial barriers must be carefully determined.[339] To take the negative side first, they could not, of course, be defended against unexpected attack, like the walls of a town, unless the assailants were only a small raiding party numbering some twenty or thirty men. On the other hand, in spite of the parallel of the ‘Customs Hedge’ in India,[340] it seems unlikely that fiscal considerations played any large part in determining the government on their construction. They doubtless acted, when built, as a check on smuggling, but the expense of their maintenance would have been quite out of proportion to the value of the trade done with the German or British tribes.
The first purpose which they served was to furnish a screen behind which patrols could march in comparative safety, both by day and night, and keep the whole line under constant surveillance. Thus the passing of a hostile force could be instantly reported by messenger or signal[341] to the nearest castella, whence detachments could at once start in pursuit. Secondly, whereas the defenders nearly always had a mounted force close at hand drawn either from the alae or the numerous cohortes equitatae, the raiders would probably be unmounted, since their start would be lost if they delayed to fill up the ditch and make a gap in the barrier large enough for their horses.[342] This barrier, too, had to be crossed again in retreat, and presented a very serious obstacle to a force encumbered with booty. Indeed, the defenders might reserve the great part of their forces for this moment, as is recommended by Byzantine military writers describing similar conditions.[343]
This sketch of the methods of defence employed applies more particularly to the German and Raetian frontiers. In Britain, more particularly on the southern line, it is probable that a more serious defence was intended. In the first place, the massive stone wall, on which the defenders could stand, was obviously stronger than anything on the trans-Rhenane section.[344] Secondly, we have noted that the garrison was stronger than in Germany, and could be more easily reinforced. Moreover, even after the final abandonment of Scotland, forts were still held in front of the southern line. Netherby on the Esk, and Habitancium (Risingham) and Bremenium (High Rochester) on Dere Street, were occupied by cohortes miliariae equitatae well into the third century, and at the last two forts we find a numerus exploratorum attached to the regular auxilia.[345] These strong outposts would have been able to check or harass the enemy’s advance and give warning to the garrison of the wall of any impending attack.