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 ofcomitatenses, 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 иloviani. 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 thestatus 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 ofcomitatenses, that are vexillations and legions, the title ofcomitatenses 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.