Between the middle of the fourth and the end of the sixth century lies a period of transition in military history, an epoch of transformations as strange and as complete as those contemporary changes which turned into a new channel the course of political history and civilisation in Europe. In war, as in all else, the institutions of the ancient world are seen to pass away, and a new order of things develops itself. Numerous and striking as are the symptoms of that period of transition, none is more characteristic than the gradual disuse of the honoured name of ‘Legion,’ the title intimately bound up with all the ages of Roman greatness. Surviving in a very limited acceptance in the time of Justinian[2], it had fifty years later become obsolete. It represented a form of military efficiency which had now completely vanished. That wonderful combination of strength and flexibility, so solid and yet so agile and easy to handle, had ceased to correspond to the needs of the time. The day of the sword and pilum had given place to that of the lance and bow. The typical Roman soldier was no longer the iron legionary, who, with shield fitted close to his left shoulder and sword-hilt sunk low, cut his way through the thickest hedge of pikes, and stood firm before the wildest onset of Celt or German[3]. The organization of Augustus and Trajan was swept away by Constantine, and the legions which for three hundred years had preserved their identity, their proud titles of honour, and their _ésprit de corps_, knew themselves no longer[4]. Constantine, when he cut down the numbers of the military unit to a quarter of its former strength, and created many scores of new corps[5], was acting from motives of political and not military expediency[6]. The armament and general character of the troops survived their organization, and the infantry, the ‘robur peditum,’ still remained the most important and numerous part of the army. At the same time, however, a tendency to strengthen the cavalry made itself felt, and the proportion of that arm to the whole number of the military establishment continued steadily to increase throughout the fourth century. Constantine himself, by depriving the legion of its complementary ‘turmae,’ and uniting the horsemen into larger independent bodies, bore witness to their growing importance. It would seem that the Empire--having finally abandoned the offensive in war, and having resolved to confine itself to the protection of its own provinces--found that there was an increasing need for troops who could transfer themselves with rapidity from one menaced point on the frontier to another. The Germans could easily distance the legion, burdened by the care of its military machines and impedimenta. Hence cavalry in larger numbers was required to intercept their raids.