[Illustration:  PLATE 18.]

Narses ascended the throne in A.D. 292 or 293.  It was at least as early as A.D. 296 that he challenged Rome to an encounter by attacking in force the vassal monarch whom her arms had established in Armenia.  Tiridates had, it is evident, done much to provoke the attack by his constant raids into Persian territory, which were sometimes carried even to the south of Ctesiphon.  He was probably surprised by the sudden march and vigorous assault of an enemy whom he had learned to despise; and, feeling himself unable to organize an effectual resistance, he had recourse to flight, gave up Armenia to the Persians, and for a second time placed himself under the protection of the Roman emperor.  The monarch who held this proud position was still Diocletian, the greatest emperor that had occupied the Roman throne since Trajan, and the prince to whom Tiridates was indebted for his restoration to his kingdom.  It was impossible that Diocletian should submit to the affront put upon him without an earnest effort to avenge it.  His own power rested, in a great measure, on his military prestige; and the unpunished insolence of a foreign king would have seriously endangered an authority not very firmly established.  The position of Diocletian compelled him to declare war against Narses in the year A.D. 296, and to address himself to a struggle of which he is not likely to have misconceived the importance.  It might have been expected that he would have undertaken the conduct of the war in person; but the internal condition of the empire was far from satisfactory, and the chief of the State seems to have felt that he could not conveniently quit his dominions to engage in war beyond his borders.  He therefore committed the task of reinstating Tiridates and punishing Narses to his favorite and son-in-law, Galerius, while he himself took up a position within the limits of the empire, which at once enabled him to overawe his domestic adversaries and to support and countenance his lieutenant.

The first attempts of Galerius were unfortunate.  Summoned suddenly from the Danube to the Euphrates, and placed at the head of an army composed chiefly of the levies of Asia, ill-disciplined, and unacquainted with their commander, he had to meet an adversary of whom he knew little or nothing, in a region the character of which was adverse to his own troops and favorable to those of the enemy.  Narses had invaded the Roman province of Mesopotamia, had penetrated to the Khabour, and was threatening to cross the Euphrates into Syria.  Galerius had no choice but to encounter him on the ground which he had chosen.  Now, though Western Mesopotamia is ill-described as a smooth and barren surface of sandy desert, without a hillock, without a tree, and without a spring of fresh water, it is undoubtedly an open country, possessing numerous plains, where, in a battle, the advantage of numbers is likely to be felt, and where there is abundant