This tendency to deterioration on the part of the Roman infantry, and the consequent neglect of that arm by the generals of the time, were brought to a head by a disaster. The battle of Adrianople was the most fearful defeat suffered by a Roman army since Cannæ; a slaughter to which it is aptly compared by the military author Ammianus Marcellinus. The Emperor Valens, all his chief officers10, and forty thousand men were left upon the field; indeed the army of the East was almost annihilated, and was never reorganized upon the same lines as had previously served for it.
The military importance of Adrianople was unmistakable; it was a victory of cavalry over infantry. The imperial army had developed its attack on the position of the Goths, and the two forces were hotly engaged, when suddenly a great body of horsemen charged in upon the Roman flank. It was the main strength of the Gothic cavalry, which had been foraging at a distance; receiving news of the fight it had ridden straight for the battlefield. Two of Valens’ squadrons, which covered the flank of his array, threw themselves in the way of the oncoming mass, and were ridden down and trampled under foot. Then the Goths swept down on the infantry of the left wing, rolled it up, and drove it in upon the centre. So tremendous was their impact that the legions and cohorts were pushed together in helpless confusion. Every attempt to stand firm failed, and in a few minutes left, centre, and reserve were one undistinguishable mass. Imperial guards, light troops, lancers, foederati and infantry of the line were wedged together in a press that grew closer every moment. The Roman cavalry saw that the day was lost, and rode off without another effort. Then the abandoned infantry realised the horror of their position: equally unable to deploy or to fly, they had to stand to be cut down. It was a sight such as had been seen once before at Cannæ, and was to be seen once after at Rosbecque. Men could not raise their arms to strike a blow, so closely were they packed; spears snapped right and left, their bearers being unable to lift them to a vertical position: many soldiers were stifled in the press. Into this quivering mass the Goths rode, plying lance and sword against the helpless enemy. It was not till two-thirds of the Roman army had fallen that the thinning of the ranks enabled a few thousand men to break out11, and follow their right wing and cavalry in a headlong flight.
Such was the battle of Adrianople, the first great victory gained by that heavy cavalry which had now shown its ability to supplant the heavy infantry of Rome as the ruling power of war. During their sojourn in the steppes of South Russia the Goths, first of all Teutonic races, had become a nation of horsemen. Dwelling in the Ukraine, they had felt the influence of that land, ever the nurse of cavalry, from the day of the Scythian to that of the Tartar and Cossack. They had come to ‘consider it more honourable to fight on horse than on foot12,’ and every chief was followed by his war-band of mounted men. Driven against their will into conflict with the empire, they found themselves face to face with the army that had so long held the world in fear. The shock came, and, probably to his own surprise, the Goth found that his stout lance and good steed would carry him through the serried ranks of the legion. He had become the arbiter of war, the lineal ancestor of all the knights of the middle ages, the inaugurator of that ascendancy of the horseman which was to endure for a thousand years.
Theodosius, on whom devolved the task of reorganizing the troops of the Eastern empire, appears to have appreciated to its fullest extent the military meaning of the fight of Adrianople. Abandoning the old Roman theory of war, he decided that the cavalry must in future compose the most important part of the imperial army. To provide himself with a sufficient force of horsemen, he was driven to a measure destined to sever all continuity between the military organization of the fourth and that of the fifth century. He did not, like Constantine, raise new corps, but began to enlist wholesale every Teutonic chief whom he could bribe to enter his service. The war-bands which followed these princes were not incorporated with the national troops; they obeyed their immediate commanders alone, and were strangers to the discipline of the Roman army. Yet to them was practically entrusted the fate of the empire; since they formed the most efficient division of the imperial forces. From the time of Theodosius the prince had to rely for the maintenance of order in the Roman world merely on the amount of loyalty which a constant stream of titles and honours could win from the commanders of the ‘Foederati.’
Only six years after Adrianople there were already 40,000 Gothic and other German horsemen serving under their own chiefs in the army of the East. The native troops sunk at once to an inferior position in the eyes of Roman generals, and the justice of their decision was verified a few years later when Theodosius’ German mercenaries won for him the two well-contested battles which crushed the usurper Magnus Maximus and his son Victor. On both those occasions, the Roman infantry of the West, those Gallic legions who had always been considered the best footmen in the world, were finally ridden down by the Teutonic cavalry who followed the standard of the legitimate emperor13.
A picture of the state of the imperial army in the Western provinces, drawn precisely at this period, has been preserved for us in the work of Vegetius, a writer whose treatise would be of far greater value had he refrained from the attempt to identify the organization of his own day with that of the first century, by the use of the same words for entirely different things. In drawing inferences from his statements, it has also to be remembered that he frequently gives the ideal military forms of his imagination, instead of those which really existed in his day. For example, his legion is made to consist of 6000 men, while we know that in the end of the fourth century its establishment did not exceed 1500. His work is dedicated to one of the emperors who bore the name of Valentinian, probably to the second, as (in spite of Gibbon’s arguments in favour of Valentinian III) the relations of the various arms to each other and the character of their organization point to a date prior to the commencement of the fifth century.
A single fact mentioned by Vegetius gives us the date at which the continuity of the existence of the old Roman heavy infantry may be said to terminate. As might be expected, this epoch exactly corresponds with that of the similar change in the East, which followed the battle of Adrianople. ‘From the foundation of the city to the reign of the sainted Gratian,’ says the tactician, ‘the legionaries wore helmet and cuirass. But when the practice of holding frequent reviews and sham-fights ceased, these arms began to seem heavy, because the soldiers seldom put them on. They therefore begged from the emperor permission to discard first their cuirasses, and then even their helmets, and went to face the barbarians unprotected by defensive arms. In spite of the disasters which have since ensued, the infantry have not yet resumed the use of them.... And now, how can the Roman soldier expect victory, when helmless and unarmoured, and even without a shield (for the shield cannot be used in conjunction with the bow), he goes against the enemy14?’
Vegetius--often more of a rhetorician than a soldier--has evidently misstated the reason of this change in infantry equipment. At a time when cavalry were clothing themselves in more complete armour, it is not likely that the infantry were discarding it from mere sloth and feebleness. The real meaning of the change was that, in despair of resisting horsemen any longer by the solidity of a line of heavy infantry, the Romans had turned their attention to the use of missile weapons,--a method of resisting cavalry even more efficacious than that which they abandoned, as was to be shown a thousand years later at Cressy and Agincourt. That Vegetius’ account