Narseh was decisively voted in favour by the majority, and guaranteed "to enter the throne of our father and our forefathers with the help of the Gods, in their name and that of our forefathers." Amongst those nobles who supported Narseh was the leading priest Kartir, which is attested in the Paikuli inscription. When Narseh ascended the throne, the eastern portion of Mesopotamia (since 244) and all of Armenia (since 252) were under Iranian rule. The traditional notion of the western part of Armenia had been given to the Arsacid prince Tiridates III has been questioned. According to historian Ursula Weber, "It is quite certain" that the whole of Armenia continued to be 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. The proposition of Narseh presumably following Shapur I's expansionistic approach does not match with his testimony in the Paikuli inscription; "And Caesar and the Romans were in gratitude (?) and peace and friendship with us." Contrary to the testimony, however, the two empires soon clashed with each other−in 296. From a Roman viewpoint, the mutual relations with Iran had been heavily strained due the aggressive and expansionistic approach of Ardashir I and Shapur I. However, the conclusive causes for the Roman offensive was possibly due to their territorial losses and the disadvantageous change in the sphere of authority and influence in the Mesopotamian-Armenian lands in the 240s and 250s. Galerius, "Caesar" under Emperor Diocletian, invaded Mesopotamia, which Narseh had occupied hoping to check his advance. Three battles were fought subsequently, the first two of which were indecisive. In the third fought at Callinicum, Galerius suffered a complete defeat and was forced to retreat. Galerius crossed the Euphrates into Syria to join his father-in-law Diocletian at Antioch. On his arrival at Antioch, Galerius was rebuked by Diocletian who disgraced him for his shameful defeat at the hands of Narseh. Vowing to take revenge, Galerius made preparations throughout the winter of 297 and invaded Armenia with 25,000 men. Supported by the Armenians, Galerius surprised Narseh in his camp at the Battle of Satala and inflicted a crushing defeat on the latter, forcing him to flee in haste.