Having analysed a maximal scope of historical sources for the time of 253–353, we may deduce that during this period Roman Empire had only two selfsufficient field (mobile) armies. The first one, which is earlier, is that established by the Emperor Gallienus around the late 250th (between 256/257 and 260), the second one, which is more late, is that of army of comitatenses established by Constantine I the Great in 312–324 but mentioned officially for the first time only in 325. All the rest troops took part in campaigns of the Tetrarchs, i. e. existed during 284–312, were only the field personal guards of the Tetrarchs, but not the self-sufficient armies as such.
These guards were private elite military retinues of each of the Tetrarchs, they consisted of different military detachments. These guards were not the armies, their effective was too small for to call them as armies. Field guards of the Tetrarchy were the groups of military units collected for the specific period of time, i. e. in order to take part in certain military expeditions.
As to self-sufficient expeditionary armies, we may say that they were permanent regional troops, i. e. they located in strictly defined regions of Empire. Army of Gallienus was a dispersed military corps, i. e. this army, in turn, consisted of some military groups, and each group occupied the relevant region. Sources on Gallienus' reign (not only narrative, but also epigraphic and numismatic) gave us a chance to trace three such military groups. One of them located in Northern Italy, its head-quarter was Milan, another stood in the province of Raetia north of the Alps, a third one resided at the Danubian frontier.
An Italian military corps was established for the first time as early as 256/257, it consisted of units called vexillations, that are military units detached from the staff of old frontier legions, we mean Rhine, Alpic, Britains and Danubian legions. All this legions have existed since early Principate times. A coin issue of Gallienus dated by 257/258 or 260/261 mentions 18 legions, which detached their vexillation to Gallienus' field corps. An original effective of this corps was nearly 10 000 men, later it was reinforced up to 15 000 men, mainly thanks to units of Dalmatian cavalry and old frontier alae and cohortes, which had served earlier at the Danubian frontier. We may consider as an additional and composite element of this corps the rapid Moorish cavalry.
Besides units, which defended North Italy (Milan military corps), we may recognize one another group of vexillations, this one served in Raetia north of the Alps — these were the vexillations of Rhine frontier legions, they were called Germaniciani. But besides Italian and Alpic expeditionary corps, an army of Gallienus included a third group located at the Danubian frontier. The third group served in upper Pannonia, it defended Danubian frontier from invasions of external barbaric tribes and from internal enemies, such as different usurpers. Inscriptions from the Roman fort of Petovio in Pannonia are our only evidence which cast a light upon an existence of this third military corps, these inscriptions mention vexillations of two old Danubian frontier legions, that are of V Macedonica and XIII Gemina.
As for vexillations themselves, we have to say that they consisted of both cavalry and infantry. Each of the vexillation comprised of five legionary cohorts, more precisely, each legion could detach up to five its cohorts in order to build a vexillation.
In 273 a hard and decisive battle at the Syrian city of Emesa gave a victory to a new Roman Emperor Aurelianus, who destroyed the forces of rebellious Palmyric kingdom. Aurelianus sent at the Near East expeditionary corps, which comprised of all Gallienic vexillations of the North Italy and Alpic front, with except for Rhine forces commanded by Gallic usurpers. An internal structure of Aurelianus' army consisted of Dalmatian cavalry, vexillations from Alpic legions (we mean II and III Italica), Danubian legions (XIII Gemina, V Macedonica, II Aduitrix, VII Claudia, IV Flavia), detachments of Moorish cavalry and also units of a mailed cavalry.
All these units striked a powerful blow at the Palmyric kingdom and reestablished a Roman control over the Near East provinces. But, at the same time, this expeditionary army finally was dismissed and dispersed over different places of Roman frontier, especially at the Danubian and Italian fronts.
Military units took part in battles against the Palmyric kingdom returned in their home (native) provinces, mainly at the Danubian front, but they served there as self-sufficient and single garrisons, without any strict addiction to their «maternai» legions.
Our sources don't let us to say about the strict and defined strength of military detachments, moreover, we can not say about the general and constant numbers for all the kinds of troops. As for effective of four regional armies of the Tetrarchy themselves, we deduced that an effective was 80 000 men per each. When Diocletian devided Empire in four large territorial parts in 293 and, therefore, established a system of the Tetrarchs (i.e. a power of the four со-rulers, two of them were the senior Augusti, and two rest persons were the junior Caesars), each of the part had its own army. All four armies were strictly the frontiers, each consisted of 80 000 men. So, a general strength of all the Roman land army during the reign of Diocletian was nearly 320 000 men.
The main military struggle, the crucial event of 293–305 was a Persian campaign of Caesar Galerius, which happened in two stages. The first stage happened in Autumn of 296, it was unsuccessfull for the Romans and ended in defeat of Galerius' army. The second stage, most important and desicive, lasted from Spring to Autumn of 298, at this time the Romans defeated Persian king Narseus within a territory of the Northern Mesopotamia, in upper Tigris and Euphrates.
A defeat of the Persians led Diocletian to the vast reorganization of the Roman defence system at the Near East and the South Caucasus. Thanks to victory over the Persians in 298 Diocletian recruited among the people and tribes, which supported the Persians, some military units and moved them in the very distant regions, such as Egypt, where they continued to locate there up to the end of the IVth c.
These were military ethnic units, their staff consisted of young men from noble families, representatives of local aristocracy of tribes and people, who supported the Persians in 298. More presicely, we mean the I ala of the Hiberi, the IXth cohorts of the Tzani and the I ala of the Avasgi. Units enumerated were composed of local aristocracy from these tribes and people.
Moreover, Diocletian located at the south coast of the Black sea, in Trebizond, a new legion, that is I Pontica, this legion had to defence the province of Pontus (Trebizond was an administratuive centre of this province) from possible incursions of the tribe of the Tzani. A garrison of the First Pontic legion, which stood in Trebizond, was an important structural element of the Roman defence system within the Armenian highland. Another important element was a vexillation of the Martenses («soldiers of Mars»), which occupied a territory of the province of Minor Armenia.
As for the Roman army itself, which defeated the Parsians in 298, we concluded that this army consisted of vexillations detached from old Danubian frontier legions, i. e. XI Claudia, VII Claudia, I Italica, IV Flavia, I Illyrica and V Macedonica. An effective of this military group was 25 000 men.
In 298–305 Diocletian allocated at the Near East front a whole series of new military units and at the same time he reinforced some old units, which had served there since early Principate. We can enumerate among old legions three frontier units, that are III Cyrenaica, I Illyrica and III Gallica. We have to add that after 298 Diocletian tranferred the Third legion of Cyrenaica in the province of Mesopotamia. Also we have to extend a list of old legions by VI Ferrata legion and X Fretensis legion, both units occupied a territory of Palestine.
Finally, a whole series of epigraphic data evidence that at the same time Diocletian formed and allocated at the Near East, mainly in Arabia Petraea and Palestine, some new alae and cohorts, among them we have to call the first new cohort (cohors prima nova), an ala from the fortress of Costia, i. e. ala located in this fortress (ala Costia), and also the second cohort of the Galatians (cohors II Galatarum).
Besides alae and cohorts, Diocletian recruited and allocated at the Near East many units of a mobile cavalry entitled equites, their full and official name was equites promoti indigenae («a local promoted cavalry»). These cavalry units were formed from the local population of Syria and Arabia, mainly from the local nomadic tribes.
Military reforms of Diocletian affected not only Near East provinces, they intensively developed in Egypt, especially after 298. Between 298 and 305 Diocletian formed and located in Egypt two new legions, that are III Diocletiana and I Maximiana, moreover, by Diocletian's order Egypt received a vexillation created in Gaul. We mean a vexillation which earlier had been an integral part of the legion II Flavia Constantia, the legion itself served in Gaul.
But military reforms managed by Diocletian in Egypt were naturally connected to the similar measures of Diocletian at the Near East frontier. Diocletian moved from the Roman-Persian frontier to Egypt some military units formed in Syria and Arabia or recruited from the Persian prisoners captured during the Roman-Persian war of 298. We have to call among these units the archers from the Syrian city of Emesa, a combined vexillation formed from a staff of two legions, that are I Illyrica and III Gallica, and finally a vexillation of Parthian archers together with the so-called III ala of the Assyrians.
All sources regarded in our book led us to conclusion that during the reign of Diocletian all military forces, which occupied the Roman Near East were principally a frontier army, they represented only the frontier military units, that are legions, vexillations, alae and cohorts. No one province had any of expeditional forces, no one province had any units from mobile imperial field guards. By other words, Diocletian didn't move at the Near East any units from his personal imperial bodyguard. The only one expeditional military group, which located and fought at the Near East during 293–305 was a military corps of Caesar Galerius composed of vexillations from Danubian frontier legions. Initially this military corps occupied Mesopotamia and then these vexillations moved to the south, in Arabia Petraea. But this group of vexillation located at the Near East frontier only during a short period of time, from Spring to Autumn of 298. After a desicive victory over the Persians Galerius' troops returned in their home provinces at the Danubian frontier. We have to stress that only two small vexillations rested at the Near East and continued to be there till the end of the IVth century, we mean vexillations of the Xlth legion of Claudia and the Vth legion of Macedonica.