According to Eutropius and Festus, two historians who wrote under the direction of the Emperor Valens in the second half of the 4th century, at a time when the Roman emperor Trajan was perceived as "a valuable paradigm for contemporary events and figures", Assyria was one of three provinces (with Armenia and Mesopotamia) created by Trajan in AD 116 following a successful military campaign against Parthia that in that year saw him cross the River Tigris from Mesopotamia and take possession, in spite of resistance, of the territory of Adiabene and then march south to the Parthian capital of Seleucia-Ctesiphon and to Babylon.

[5] Trajan overcame the revolt, capturing and burning Seleucia and Edessa, and even setting up a puppet Parthian king; but then, on his journey homeward in triumph, he fell sick and died on 8 August 117.

He sent the puppet Parthian king elsewhere and restored to the former ruler the lands east of the Euphrates, together with his daughter who had been captured, preferring to live with him in peace and friendship.

[8] The fourth-century historians Eutropius and Festus assume that the supposed Roman province of Assyria was situated "east of the Tigris and usually identified with Adiabene".

[12] Following his successful campaign, Septimius Severus instituted two new Roman provinces: Mesopotamia and Osroene, a kingdom founded in the 2nd century BC,[13] centered on Edessa.

[14] Roman influence in the area came to an end under Jovian in 363, who abandoned the region after concluding a hasty peace agreement with the Sassanians and retreating to Constantinople to consolidate his political power.