Perso-Roman wars of 337–361
They were a result of long-standing competition between the rival powers over influence in the border kingdoms of Armenia and Iberia, as well as the desire of Shapur II, after his Arab campaign, to revoke the unfavorable terms of the Treaty of Nisibis, which had concluded the previous war between the empires.
The attack which was launched, supported by Sasanid war-elephants,[13] bogged down in the muddy waters formed by the lake, and Shapur gave the signal to fall back.
In the interval of a single night the defenders effected the repair of the walls,[13] this along with news of Hunnic invasions of the eastern provinces, compelled Shapur to a cessation of hostilities against Rome.
Yet, he dispatched civil and military officers accompanied by a sophist on a new embassy to the Sasanid court, advising Shapur II of the necessity of more reasonable grounds for peace.
[20] With most of the eastern tribes (including the Chionites) now supporting his army, Shapur crossed the Tigris at Nineveh and marched to Singara which he stormed or blockaded.
[23] As soon as Ursicinus' spies, including later historian Ammianus Marcellinus,[24] had informed him the Sasanian army had crossed the Tigris, he secured the defenses of Nisibis and proceeded to Amida.
After giving orders for the fields of Roman Mesopotamia to be burned to prevent Shapur's army using it as fodder, the population took refuge in the fortified city.
With Shapur's crossing of the Tigris secured, Ursicinus fell back to Amida, from which he could harass the Sasanians if they besieged Nisibis or threaten their lines of communication and supply in case they marched west.
Six Roman legions: V Parthica, XXX Ulpia Victrix, Magnentius, Decentius, Superventores and Praeventores and a detachment (vexillation) of X Fortenses had been gathered at Amida and took up its defence.
Faced with this predicament, Antoninus advised the Sasanid monarch to force march his army and cross the Euphrates northwest of Amida.
Marching by way of Horre, Miacarire, Carcha, and the fortresses of Reman and Busan which capitulated on his approach, Shapur arrived beneath the walls of Amida at the end of July.
But before the king could make known the reason of his approach, a dart from the Roman ballistae struck his son and crown prince killing him instantly.
[17] The Sasanids encircled the city on every side; as auxiliaries, the Vertae were assigned the assault of the south wall, the Albanians on the north, the Chionites to the east, and the Segestanis with their war-elephants on the west.
The Sasanids then set themselves to raising mounds for scaling the walls, and siege towers captured from the Romans in the previous war at Singara.
Ursicinus, who proposed a bold attack using light infantry to divert the Sasanid blockade, was accused of fomenting a treacherous reverse through his excessive zeal for the relief of the city.
[36] He lingered only to repair the walls and post a strong garrison in Bazabde before he proceeded on an expedition against Virtha, a formidable fortress on the outliers of the Mesopotamian desert.
Unwilling to resume the blockade of Bazabde and fearing a costly siege, he sent his two generals, Arbetio and Agilo, to observe the motions of Shapur from the further bank of the Tigris to inform him when the latter should cross the river.
[42] On account of the immediate Sasanid threat, Constantius was unable to directly respond to his cousin's usurpation, other than by sending missives in which he tried to convince Julian to resign the title of Augustus and be satisfied with that of Caesar.
