Narseh then moved south into Roman Mesopotamia, where he inflicted a severe defeat on Galerius, then commander of the Eastern forces, in the region between Carrhae (Harran, Turkey) and Callinicum (Raqqa, Syria).

According to historian Ursula Weber, "it is quite certain" that the whole of Armenia continued to a part of the Sasanian Empire in the 3rd-century, until it was later ceded to the Romans in 298/9 after the Peace of Nisibis.

Local aid also gave the Romans the advantage of surprise over the Persian forces, and, in two successive battles, Galerius secured victories over Narseh.

[9] According to the account of Faustus of Byzantium (in Armenian: P'awstos Buzand): Narseh was himself wounded in the battle and fled Armenia into the Persian heartland, presumably in the hope of raising more troops; the booty taken by the victors was so great that it achieved legendary status and its transport back to Roman territory posed a logistical challenge.

[10] Narseh's wife, Arsane, would live out the remainder of the war in Daphne, a suburb of Antioch, serving as a constant reminder to the Persians of the Roman victory.

[11] Galerius advanced into Media and Adiabene, winning continuous victories, most prominently near Resaina (Ras al-Ayn), and securing Nisibis (Nusaybin) before 1 October 298.

Rome itself received a part of Upper Mesopotamia that extended even beyond the Tigris - including the cities of Tigranokert, Saird, Martyropolis, Balalesa, Moxos, Daudia, and Arzan.