During the so-called second Tetrarchy (305–324) the Emperor Constantine I established an expeditionary (field) army, which originally quartered in Gaul and then was moved in Illyricum and finally in the Lower Danube region, that is in Thrace (324), where it continued to be until 353.

These expeditionary forces were composed in Gaul, its ground and main essence were vexillations detached from Rhine frontier legions. At the same time epigraphic data enabled us to maintain a structure of military corps, which was an enemy of Constantine I in 312 — we mean a personal guard of Maximianus Herculius and his son Maxentius, in 306 this guard deserted to Maxentius and supported his usurpation. Among the units adherent to Maxentius were elite vexillations equites promoti, comites domini nostri, Moesiaci, Minervii, Moorish cavalry and also a detachment of spearmen (lanci arii). Apart from units enumerated above, Maxentius had a support from a garrison of the city of Rome, that are praetorian guard and units of equites singulares Augusti (a cavalry imperial guard formed as early as during the Principate times).

So, when Constantine I defeated Maxentius, a victorious Emperor destroyed a usurper's guard in part — firstly, we mean the praetorian cohorts and the units of equites singulares Augusti, which were immediate instigators of Maxentius' plot. Also we assume that Constantine abolished a corps of spearmen (lanciarii).

However, we think that the vexillations of Minervii and Moesiaci still existed after 312, Constantine simply moved them from Italy to Rhine frontier and deprived them of their elite status (rank) of palace units, they were modified into regular frontier garrisons.

Another field guard (the so-called comitatus) defeated by Constantine was a guard of Licinius, this one was a very fluent and incoherent military group, Licinius' field guard existed as a single and united composition only during and for the military campaigns. After completion of a military activity the comitatus of Licinius was dispersed and stopped to exist. The crucial element of Licinius' military forces were frontier units (legions, vexillations, alae and cohorts) of the Danube regions. These units stood not in the ruler's residence, but in different fortresses at the Danube frontier, so, they were scattered along the entire course of the river.

Inscriptions from Balkan and Danube regions enabled us to trace that vexillations under Licinius' command were mostly of two kind, that are detachments of shield bearers (scutarii) and archers (sagittarii), also Licinius commanded over units of mailed cavalry (catafractarii).

As to field army of Constantine I, we turned to epigraphic data in order to make clear an internal structure of this army. For the most part expeditionary forces of Constantine I established in Gaul consisted of two kinds of military units.

The first one are mercenary ethnic detachments composed of the Germans. As anybody knows, Constantine I managed some military campaigns at the Rhine frontier against the German tribes, but Constantine I also signed treaties with some of these tribes about the military assistance and alliances from the part of the Germans in favor of the Romans. According to these treaties, the Germans sent its military retinues in army of Constantine I, their service was paid by money and as a result of this they occupied a privileged place within Constantine's army.

The second one were regular vexillations levied from internal (local) citizens of Empire, mainly from the populace of Gaul. First of all, inscriptions mention about a vexillation detached from the legio II Italica, this vexillation quartered in the fortress of Divitia, within modern Köln, and was named Divitenses after the place of its garrison. Apart from Divitenses, Constantine I commanded over the vexillation of legio IV Flavia (the legion itself located in upper Moesia), a mailed cavalry, Dalmatian cavalry and two self-sufficient legions, that are loviani и Herculiani, they had stood in Gaul since the first Tetrarchy. Latin panegyrics report that the strength of Constantine's army was nearly 40 000 men.

Among Constantine's vexillations were also two units of mailed cavalry from Poitiers and Amiens and vexillation entitled as Ursarienses after a name of the city of Ursaria, which located in the Gallic province of Maxima Sequanorum (at the south-east of Gaul, near to borders between Italia and Raetia).

All vexillations enumerated above were sent from Gaul in Western Illyricum in 316 for to fight against Licinius. Therefore, an army of Constantine I arrived in Western Illyricum and then located there from 316 to 324 (precisely in Pannonian provinces, Upper Moesia and, possibly, in Dalmatia). Afterwards, in 324, when Constantine I decisively won over Licinius, this military group with its full strength advanced down to Lower Danube, in Thrace, which was a place for battles between Constantine I and Licinius.

However, during the second Tetrarchy, in 305–324, military reforms touched upon not only Balkan areas, but also the Near East regions. As we suppose, Maximinus II Daia, Caesar and then a self-proclaimed Augustus, who reigned in Eastern provinces of Empire during 305–313, managed a vast reform, which resulted to an establishment of a self-sufficient and single province of Minor Armenia and creation of a new office, that is dux of Minor Armenia, this reform enabled Maximinus to reinforce a defence of the Near East front from Persian invasions. The evidence of many hagiographie texts gave us a chance to maintain that Maximinus managed his reform during 305–312, a territory of Minor Armenia was singled out of Cappadocia and modified into a self-sufficient province, the head of this province was a special military official, we mean dux, so, three powerful legions stood under his supervision, that are XV Apollinaris, XII Fulminata and I Pontica.

We suppose that it was dux of Minor Armenia who headed and managed a huge military operation by order of Maximinus II Daia, this campaign was deployed on the territory of Great Armenia, an independent state under the ruling of the Arsacid dynasty, Great Armenia immediately bordered to Minor Armenia and lied east of Minor Armenia.

We dated this campaign by 312–313, in order to study the course of events we turned to Syrian and Armenian historical chronicles translated into modern West European and Latin languages. According to our point of view, in 312 the tribes of the Huns invaded in Great Armenia by instigation of the Persians, these Huns lived north to Caucasus mountain chain, obviously within modern Dagestan in Russia, they crossed the mountain passes and arrived in Great Armenia.

So, when the Huns invaded in Great Armenia and began to damage its territories, the king of Great Armenia Trdat III, who came to power by an assistance of the Romans as early as 298, applied to Maximinus II Daia for military help, in response Maximinus sent to Armenia a military corps, which consisted of three legions, that are XV Apollinaris, XII Fulminata and I Pontica.

In sum, dux of Minor Armenia and legions subordinated to him really held military action on the territory of Great Armenia, but not against Great Armenia, because their enemy was the Huns, who ravaged Armenian lands and could invade further in Roman lands.

We proposed that the first phase of a military campaign against the Huns happened under Maximinus II Daia in 312, in any case, before the winter of 312–313, but these military actions were successfully ended only by the new Emperor Licinius, who took control over Asia Minor in the spring of 313. At the same time, epigraphic data, hagiographical sources and administrative treatises enabled us to conclude that the Near East army of Maximinus, as in Minor Armenia, was composed only from the frontier units, that are vexillations, legions, alae and cohorts.

As we have noted above, in 324 an expeditionary army of Constantine I was transferred in Thrace at the Lower Danube, and next year this army received a high-ranking status of comitatenses, i. e. «those who follow the Emperor». However, a situation at the Near East front was not in the best conditions. Epigraphic data from Syria, Mesopotamia and Arabia enabled us to trace that during the last years of Constantine I's life, especially between 330 and 334, a defence of these regions was supported exclusively by the small frontier garrisons, alae and cohorts, which located in very scattered fortresses, these fortresses were removed far from each other by a long distances, and as a result, one garrison could not arrive and help in time to another if an external enemy launched an assault. Moreover, we concluded that the Near East frontier featured by a full absence of any units of expeditionary army up to 337, i. e. to a death of Constantine I. The first partial relocation of Constantine I's Danube expeditionary army of comitatenses happened only in 337, the last year of Constantine's reign. Then, in the spring of 337, Constantine marched against Persia and took with him some military units from the army of comitatenses, the Emperor moved them in Bithynia.

We assume that military detachments which located in Bithynia in the spring of 337 as equally as all the rest part of the Danube expeditionary army which continued to be in Thrace, came under the power of Constantius II, a middle son of Constantine I, this event happened already in the summer of the same year. Up to the summer of 337 Danube frontier didn't subordinate to Constantius because in 335 Constantine I gave Balkan-Danube regions under the government of his nephew, Dalmatius the Junior, and it's natural that Thrace also was among the territories which came under Dalmatius' jurisdiction. We need to point out that Thrace was a residence-place for the army of comitatenses, so, between 335 and 337 the Danube army of comitatenses was headed strictly by Dalmatius the Junior.

Respectively, at the beginning of June of 337 (we follow the dating of R. Burgess) Constantinople became a place for bloody palace plot, which was supplied by murders and violation, a supposed instigator and promoter of this plot was Constantius II, and its victims were Hannibalian, another nephew of Constantin I and a brother of Dalmatius the Junior, and Dalmatius himself. This political crisis resulted to extension of territorial power of Constantius II, because he gained a power over lands which earlier obeyed Dalmatius the Junior, precisely, these were provinces along the Lower Danube, Thrace, Minor Scythia and the Lower Moesia. So, Constantius II maintained his control over Danube expeditionary forces, which located primarily in Thrace.

As a result, to the summer of 337 Constantius ΙΓ troops included two military groups, the first one located at the Near East frontier and was a frontier army (alae, cohorts and vexillations), the second one resided in Thrace and was an expeditionary army of comitatenses. However, we have to point out that it was Constantius II who first relocated essential part of Danube expeditionary forces from Thrace to the Near East. We supposed that between 337 and 342 Constantius II moved many units of Danube army in Syria, Mesopotamia and Arabia.

Inscriptions from Syria, Arabia and Mesopotamia attest a presence of some vexillations within these provinces, and previously, before their relocation to Near East front, these vexillations strictly posted at the Danube front and were integral parts of the army of comitatenses, we mean primarily the units of Aureliaci, Mauri Illyriciani Constantes, a regiment of Dalmatian cavalry and a band of recruits levied from Thrace.

But, at the same time, the evidence of Armenian historian Pawstos Buzandos and Moses Khorenats'i enabled us to conclude that around 338–339 Constantius II moved at the Near East, mainly in Mesopotamia, the same units which earlier, in the spring of 337, had located in Bithynia. The reason of this transfer was a Persian invasion in Mesopotamia in 337–338, the Persians captured some principal cities and then advanced in Great Armenia, occupied this kingdom and deposed its legal king Tiran.

Constantius Il's response was a rapid and effective — according to the Armenian historians named above, the Emperor composed two large military bodies (corps), one was formed in Cilicia (by another words, in Isauria, at the south-east of Asia Minor), and another was levied in Galatia, at the north-west of Asia Minor, in close proximity to Bithynia.

Apparently, Constantius II sent in Mesopotamia and then in Great Armenia only one military group, in contrast to Armenian sources we think that this group included those regiments of the army of comitatenses, which stood in Bithynia in the spring and summer of 337. When Constantius II took control over Bithynia and the whole Asia Minor in that summer and gained a power over the troops of Bithynia, he decided to send these troops in Mesopotamia and Great Armenia. A result of this movement was a brilliant success of Constantius II, he managed a military campaign in Great Armenia around 338–339, defeated the Persians and drove them out of the Armenian kingdom. An Armenian ruling dynasty of the Arsacids was returned home and restored at the native throne.

However, in 350 Constantius II faced to new problem, that was a revolt of Magnentius, a prominent general in Gaul who commanded over two elite legions, Herculiani и loviani. Magnentius deposed and killed a legal Emperor Constans, who ruled in West Roman provinces and who was a junior brother of Constantius II, so, when Constantius learned about this umest, he concluded a temporal peace treaty with the Persians on a presumption of keeping the status quo. Then Constantius II went in Thrace and further in Gaul in order to destroy Magnentius.

A law issued by Constantius II three years earlier, by 347, testifies two main elements of the Danube army of comitatenses, that are vexillations and legions, the title of comitatenses is clearly mentioned in this law. This imperial constitution cast a light upon internal structure of the Danube army and attests its presence in Thrace around 347. Obviously, up to 347 an effective of this army was not more than 40 000 men.

In sum, to the late of 350 Constantius II commanded over three military bodies (groups), the first one were units moved from the Near East front, the second one were legions and vexillations of the Danube army of comitatenses and the third one were the frontier garrisons of the Western Illyricum.

According to Iohannes Zonaras, the whole effective of Constantius Il's troops was 80 000 men, and Magnentius' troops numbered nearly 36 000, a decisive battle happened in September of 351 at the city of Mursa in Pannonia, the losses were 30 000 from Constantius II and 24 000 from Magnentius, this battle was successful for Constantius II.

Primarily, we have to point out that Magnentius' army included two kind of troops, that were mercenary units levied from the Franks and the Saxons and regular legions which stood at the Rhine frontier. Epigraphy data helped us to distinguish among these regular Rhine regiments the units of Herculiani, loviani and Moesiaci. To our point of view, these regiments didn't participate in the battle at Mursa because Magnentius preferred to stay them in Northern Italy, where they served as the integral parts of a usurper's personal palace bodyguard.

When Constantius II returned to himself a power over the Northern Italia in September of 352, he didn't disband these elite regiments, but, in contrast, the victorious Emperor preferred to include them in its own army. We think that strictly in that time, around 352–353, Constantius II took and managed an important reform of conscription system for completion the army, this reform generated a vast number of new military detachments entitled as seniores and iuniores. All survived epigraphy data and evidence of narrative sources enabled us to trace that Constantius II firstly included in his army the regiments of Herculiani, loviani, and also units of Moesiaci and equites Cornuti and then he implemented them with the groups of new recruits.

Little time after Constantius II singled out these groups of recruits and modified them into self-sufficient units, which received an additional nickname of iuniores, i. e. «the juniors», precisely, Herculiani iuniores, loviani iuniores, Moesiaci iuniores and equites Cornuti iuniores. At the same time, the old «paternal» units, which saved their original staff and which were deprived of new recruits, received an additional nickname of seniores, «the seniors», that were Herculiani seniores, loviani seniores, Moesiaci seniores and equites Cornuti seniores. The main ground of om arguments was an anonymous military treaty conventionally entitled as De rebus bellicis.

To sum up, all the data under consideration enabled us to conclude that up to 353, when Constantius II won over Magnentius, the whole Late Roman military forces had only one expeditionary army, that is the Danube, which located and served in Thrace, but, as a result of a civil war between Constantius II and Magnentius this army temporarily stood in two Western regions, we mean Gaul and the Northern Italy.

So, up to 353 all the rest regional armies of Empire were only the frontier troops, they consisted of permanent and immobile frontier garrisons. These garrisons were manned by alae, legions, vexillations and cohorts. We may ascribe to frontier troops the armies of the Rhine, Near East and Illyrian fronts. Respectively, the units of army of comitatenses might serve at the Rhine, Near East or Illyrian front only temporarily, on occasion of some military campaign in one of these regions, but their permanent place for billeting was Thrace. To om point of view, an approximate number of this army stayed without serious changes, that is 40 000 men.