[337-363 A.D.]
According to Roman assertions, the Persians began the war by an invasion of Mesopotamia. Constantine died before he could take the field against them (the 22nd of May, 337). But the king’s great preparations date from the year which begins with the autumn of 337. On the first and longer half of the war, which lasted with many vicissitudes and long pauses for twenty-five years, our information is but scanty. On parts of the second, on the contrary, we possess very full reports by contemporaries and even eye-witnesses. The king’s object was to deprive the Romans of their possessions on the upper Tigris, where it must have been exceedingly inconvenient for the Persians to have them on account of their nearness to Ctesiphon. Above all, he aimed at taking the strong fortress of Nisibis; and he further desired to bring Armenia, that old apple of discord between the Eastern and Western empires, into subjection to himself once more. Three times he closely besieged Nisibis (in the years 338, 346, and 350), but in vain. Sieges, on the whole, play a very great part in this war.
If Sapor did not in the long run succeed in gaining great advantages, it was through no merit of the emperor Constantine, who was invariably defeated when he took command in person, as, for example, in the famous battle by night at Singara (Shingar, Arabic Sinjar) (348 A.D.). The main reason was that the great emperors Diocletian and Constantine had put the fortresses into admirable condition and taken other excellent measures for the protection of the provinces exposed to attack. It was a great thing gained that the Persians, even when victorious, could hardly penetrate into western Mesopotamia. Moreover the king’s forces were not large enough for him to leave garrisons in all the fortresses which he took. Thus in 360 Amida (Amid), which Sapor had taken after a long siege and with heavy loss in the previous year, was found by the Romans unoccupied. The Romans were also favoured by the circumstance that the king was at the same time engaged in conflict with several barbarous tribes. The third siege of Nisibis had almost come to a successful conclusion when he was obliged suddenly to depart to Khorasan, where his presence was urgently required.
The wars in the East brought about a long truce (from 350 to 358), interrupted only by small predatory excursions. But by the time negotiations were opened on the Roman side (356-358) Sapor had concluded peace with his enemies in the East, and offered terms which it was quite impossible to accept. In 359 and 360 hostilities were resumed with energy, and Sapor took several important fortresses. Another interval of repose ensued; but in 363 a change came over the whole conduct of the war.
[363-379 A.D.]
Vigorous, ambitious, and proved in arms, Julian, now sole emperor, determined to follow the example of Trajan, Septimius Severus, and Carus, and march straight upon the enemy’s capital. On the 5th of March he left Antioch, went first to Mesopotamia, and thence proceeded rapidly down the Euphrates. He ravaged Persian territory with fire and sword, took several cities after a short siege, among them Mahoz Malka, one of the royal cities close to Ctesiphon. He even reached Seleucia; but realising that he was not able to take the strongly fortified city of Ctesiphon on the far side of the Tigris by storm, he turned to retreat along the left bank of the river. Here for the first time Sapor’s troops began to annoy him seriously. None the less he would certainly have led the army back into Roman territory without heavy loss, but he was mortally wounded in an engagement on the 26th of June, 363.
Jovian, who was chosen emperor by the army after Julian’s death, was by no means equal to the difficult position in which he found himself, and conducted both the war and the negotiations in such a manner as ultimately to bring about a shameful peace. After the death of his dreaded enemy, Sapor behaved with equal adroitness and moderation. He obtained the retrocession of the districts to the left of the Tigris, which Galerius had won, and part of Mesopotamia, including Nisibis and Singara. The Romans with much difficulty secured permission for the inhabitants of these cities to depart elsewhere. The cession of Nisibis was the heaviest blow of all, for in all subsequent wars it was a strong point of departure to the Persians for offensive and defensive purposes.