The reconstitution of the army, started by Gallienus, played a vitally important part in the recovery of the empire:
By c. 200 the old differentiation of recruitment between legionaries and auxiliaries had disappeared or become nominal. . . . Since there were numerous legions in the east … local recruiting was larger still …. Consequently the predominant element in the Roman army came from the warlike population of the Danubian provinces.
The earlier idea of an indeterminate, variable, invisible frontier protection zone [came to be replaced] by the doctrine of fixed and fortified barriers. . .. [But] Gallienus, threatened not only by all manner of external invaders but by the dissident western empire of Postumus, took the important step of creating a field army of cavalry, which was intended to serve simultaneously as a reserve and a mobile striking force. Its principal base was Mediolanum [Milan], located at a convenient distance from the frontiers and Rome alike. This strategic centre, rapidly becoming even more important than the capital, was joined to Aquileia, Verona and Ticinum [Pavia] in a new system of north Italian defence necessitated by the loss of the upper Rhine-upper Danube area.
But the new plans differed from the old static protection because they were conceived in terms not only of fortresses but of the newly created cavalry army. This elite force consisted of squadrons (we do not know how many) which were mostly five hundred men strong. They included heavy Persian-style cavalry, looking like knights of the Middle Ages in their conical Iranian helmets, which the Germans later inherited; and an almost medieval concept of knight- hood was to be seen in the hereditary gold ring granted to the sons of its centurions. Other elements in this army were Osrhoenian and Palmyrene mercenary archers on horseback, javelin-throwing Mauretanian riders, and a novel and valuable corps of mounted Dalmatians whose Illyrian origins guaranteed loyalty to Rome and leavened the exoticism of the other contingents.
This new arm of the service was celebrated by coins of Gallienus displaying the winged horse Pegasus, to whom a dedication is offered as the spirit of alertness (ALACRITATI). Other slogans speak of the courage of the cavalry (VIRTVS EQVITVM); and there are appeals to their loyalty (FIDEI EQVITVM) [see also below] …. Yet the commanders of this formidable cavalry army, necessarily men of ability, were under great temptation to revolt ….
Aurelian … employed his expert knowledge to operate light horse successfully against the massive mailed cavalry of Zenobia. Nevertheless, he also strengthened his own heavy cavalry on a large scale. . . . Diocletian proceeded to a military reorganisation of far-reaching variety and scope. Pursuing his predecessors’ concern with mobile formations, he not only created a new barbarian mounted bodyguard (scholae), but made the field force into one of the two major parts into which the entire army was now divided. . . . Diocletian [also] reverted to earlier preoccupations with frontier defence …. Whereas Septimius’ army [193-211] had totalled between 300,000 and 400,000 men, Diocletian’s consisted of 500,000 or even more …. He was … particularly eager to make use of the warlike tastes and varying specialist skills of barbarian tribesmen. The soldiers mobilised from this almost inexhaustible source of supply included . . . Anatolians, who were to be the backbone of Byzantine armies; and many Germans.
So even in the unprecedented and apparently desperate circumstances with which he was faced, Gallienus had found time to give a new shape to his army. The Romans had long since made use of mounted javelin-men and archers, and for over a century past they had also employed certain heavily armoured units of horse. But now the formidable heavy cavalry with which the Persians and Sarmatians were confronting them demonstrated that this branch of the army needed extension on a very large scale. And so Gallienus took the significant step of creating a major cavalry corps (264-8) [partly intended, too, to stave off usurpers]. This corps, a very expensive institution since a horse’s feed cost as much as a soldier’s rations, was intended to serve not only as a striking force but as a central military reserve, which had so long been lacking until [Septimius] Severus [193-211] made a start by his expansion of the military establishment in Italy at the end of the previous century. The principal base of the new army was Mediolanum [Milan]. Located at a convenient equidistance both from the frontiers and from Rome, this centre rapidly assumed even greater practical importance than the venerable capital.
The coins of Gallienus appeal to various virtues of the new elite force … and in particular [as we have seen] to its loyalty …. [O]ne of the large gold medallions, which it had become customary to hand out to high-ranking officers as personal rewards, was now bluntly inscribed ‘Because you have remained loyal’ (OB FIDEM RESERVATAM). In order to keep the officers of the new cavalry corps in this blessed condition, they were enrolled by Gallienus, together with a number of other officers, in a select staff group of household troops (protectores domesticz), who encamped in the proximity of the emperor himself, and were attached to his own person. Nevertheless, it was precisely over this matter of loyalty that the new army reform proved most vulnerable.
Gallienus’s mobile army was 30,000 strong. Diocletian (284-305), as we have seen, also reformed the army.
As part of a large-scale reorganisation in many fields, he [Diocletian] and his fellow-Tetrarchs extensively overhauled the entire structure of the army. Pursuing his predecessors’ interest in mobile formations, he created a new barbarian mounted bodyguard, named scholae palatinae after a portico in the palace where they awaited imperial orders. These scholae were incorporated into one of the two major branches into which the entire army was now divided, the field force (comitatenses, ‘soldiers of the retinue’). This mobile force, of which each of the four rulers controlled his own section, included infantry units, but it was in cavalry that its particular strength appeared. The second major division of Diocletian’s armed forces was the frontier force (subsequently known as limitanei or riparienses), stationed along the strengthened fortifications of the borders.
The total strength of the Roman army was now half a million, perhaps about 20 per cent larger than the army of [Septimius] Severus a century earlier. It was recruited by systematic annual conscription among Roman citizens. But extensive use was also made of the warlike tastes and various specialist skills of barbarian tribesmen. These included numerous Germans, as well as men from the highlands of Asia Minor. . . . Diocletian also reorganised the navy, adding a number of small provincial fleets.
The corps of limitanei, directing much improvised fortifications, consisted largely of Germans, with the addition of other barbarians of military qualities, and drafts of Roman conscripts, and mountain people from Asia Minor. This greatly increased army had to be paid for, and Diocletian introduced forcible measures to do so. The measures taken by Diocletian were carried still further by Constantine. Subject to criticism, despite his military talents, is Constantine’s reorganisation of the army. From now on, it was divided between a frontier force and a striking force. There had been signs of this division before, but Constantine made it definitive. And there are reasons to suppose that it was a mistaken and disastrous decision, which helped to let the Germans in. Another thing that let them in was the increased admission of Germans into the army, both as top generals and as very numerous rank-and-file soldiers- again not a new development, but again they appear on an unprecedented scale. And, above all, Constantine enlarged and extended earlier policies that allowed German civilians to immigrate into the empire in thousands.
Illyrian generals were also powerful during this period.