The Battle of Samarra took place in June 363, after the invasion of Sassanid Persia by the Roman Emperor Julian. A major skirmish, the fighting was indecisive but Julian was mortally wounded in the battle. The Romans, stranded deep in Persian territory and suffering from a lack of supplies, were forced to accept terms for peace.
Julian invaded the Sassanian Empire with a force of 95,000 men, hoping by his victory to change the balance of power in the east, where the Romans had suffered recent reverses under Constantius II, and perhaps to replace Shah Shapur II with his brother Ormisdas. According to a modern historian, Julian made two blundering mistakes at the very onset of the invasion. He split his force into two, one under his cousin Procopius numbering 30,000 men, which went to northern Mesopotamia, and one numbering 65,000 men under his own leadership. His second mistake was not defeating the main Sassanid army before attacking the capital, which would eventually lead to the expedition's failure in 363. Julian at first won a tactical victory outside Ctesiphon, but could not take the city. Even worse, Procopius failed to join him with his army, due to the treachery of Arshak II, Christian king of Armenia, who was to second the movements of the northern contingent. Julian thereupon fired the fleet which he had brought down the river until Ctesiphon, and such of the baggage as could not be carried, leaving a bare three weeks' supply. He then directed his march inland into the heart of Shapur's dominions, hoping to force a battle. Several of the Christian writers who chronicled the events, as well as the historian Ammianus, blamed this decision for the subsequent disasters.
David S. Potter suggests that Julian's main faults was that he made a very risky campaign and that he didn't bring an adequate siege train. Therefore, he had to retreat after realizing that Ctesiphon was too strongly defended to be taken by assault while his army was running out of supplies. According to Gibbon, the provisions which could be afforded by the fleet were in any case insufficient, and Julian expected the more fertile inner provinces of Persia to yield his army an ample supply of fodder and forage. However, the absolutist Shapur implemented a ruthless policy of devastation of the soil, firing crops, households and provisions wherever the Romans directed their march, so that the army was soon straitened for lack of sustenance. Julian, realizing that his army could not be resupplied or reinforced, tried to force a set piece battle with his enemy, but Shapur eluded him, and his guides only misdirected and confounded his steps.
Thus the reluctant decision was taken to retreat through the district of Corduene to the north, where there was hope to find adequate supplies. After a few days of advancing through the enemy country, despite defeating Persian skirmishes and inflicting them heavy losses in the Battle of Maranga, the demoralized army, which had intended to topple the Persian monarchy, was almost depleted of provisions and exhausted by the heat of the climate and the continuous fighting.
After three quiet days, the Roman army was ambushed during its cautious advance in square formations through sloping country south of the town of Samarra . The battle began as a Persian skirmish, against the rear guard of the Roman column. A large body of cavalry and elephants then fell on the center of the Roman left wing, commanded by Anatolius. According to Ammianus Marcellinus's narrative, Julian hastened to rally his forces against the Persians, without wearing his armor, which he had removed due to the heat. He managed to raise their morale and repulse the main onset of the enemy on the left but his personal guard was dispersed during the fighting and Julian was pierced in the side by a spear, which penetrated to his liver. The spear was most probably thrown by a Saracen (Lakhmid) auxiliary in Persian service, as his doctor Oribasius concluded. The mortally wounded emperor was thrown from his horse and removed fainting from the battlefield. The struggle continued indecisively until the darkness of the night put an end to the fighting. The left wing of the Roman army was defeated, and Anatolious slain. But elsewhere the Persians were routed, their elephants and massive troops of cavalry discomfited, and their noblest generals slain. According to one's view, the Romans might be called victorious  or be said to have been defeated by the Persian forces. Julian died from his wound, in his tent, at midnight, his officers gathered around him.
The report that his dying words were "Νενίκηκάς με, Γαλιλαῖε" ("Thou hast conquered, Galilean"), supposedly expressing his recognition that, with his death, Christianity would become the Empire's state religion has often been attributed to Julian. However it actually originates much later with the derisive account of his death by Theodoret in Ecclesiastical History, Book III, Ch. 20 (c. 429), as an exclamation he made upon being fatally wounded; no prior account of such an declaration exists, even among those writers most hostile to Julian and his policies.
Libanius in his orations commemorating the life and deeds of the last legitimate pagan Roman Emperor, initially stated that Julian was assassinated by a Christian who was one of his own soldiers, but later stated that the assassin was a Saracen, or Persian soldier. Ammianus Marcellinus, Julian's chief biographer and highly valued and praised historian of 354–378, also sheds doubt that a Christian was the guilty party and echoes Libanius' later thoughts that an unknown Persian committed the deed that cost Rome dearly.
Julian had deliberately refrained from naming a successor, and the commanders assembled at dawn for the election. The honor was extended to the prefect Salutius, but he refused. Their choice then spontaneously settled on Jovian, a dissolute but amiable commander of Julian's domestic guard, whose father had been a general of distinguished merit in the same service. He at once resumed the retreat along the east bank of the Tigris, under continuous harassing from the Persians, and suffered heavy losses in successive engagements at Samarra and the camp at Carche, in which the Persians, reinvigorated by the news of Julian's death, were barely held off. After four further days of struggling on, the demoralized army finally came to a halt at Dura, where they attempted to construct a bridge to cross the river, but failed to do so, and were surrounded on all sides by the Persian army. Jovian clearly saw that the situation was now desperate. Unexpectedly, the envoys of Shapur II. arrived in his camp bearing offers of peace, and Jovian, who during the halt had exhausted his provisions, grasped eagerly at any venue of extricating the army from its dire situation. Thus he was forced to accept humiliating terms from Shapur, in order to save his army and himself from complete destruction. The treaty with Shapur, revoking the favorable conditions which Diocletian had extorted from Shapur's grandfather Narseh, surrendered Eastern Mesopotamia, the five provinces beyond the Tigris: 1) Intilene 2) Zabdicene 3) Arzanene 4) Moxoene 5) Corduene, and Armenia, as well as fifteen fortresses, including the strategic cities of Nisibis and Singara, without their inhabitants. A thirty-year truce was established between the rival empires. The loss of the elaborate chain of fortifications founded by Diocletian severely hampered the empire's defensive system in the east and gave the Persians a definite advantage in their subsequent confrontations with the Romans.
- Shapur II (iranicaonline)
- War and Warfare in Late Antiquity (2 vols.): Current Perspectives. BRILL. 2013-08-23. p. 62. ISBN 9789004252585.
- Libanius, Epistulae, 1402.2
- Browning 2004, p.196
- Edward Gibbon, The Decline and Fall of the Roman Empire, (The Modern Library, 1932) chapter XXIV., p. 822
- Potter 2004, p.518
- Gibbon, chap. XXIV., p. 822, 823
- Ammianus Marcellinus, Res Gestae, 24.8.5
- Ammianus Marcellinus, Res Gestae, 25.1.19
- Ammianus Marcellinus, Res Gestae, 25.1.7
- Gibbon, p. 825
- Browning 2004, p.213
- Gibbon, p. 827
- Ammianus Marcellinus, Res Gestae, 25.3.2-12
- Gibbon, Ibid.
- Libanius, Orations, 24.6
- Gibbon, Ibid.
- Gibbon, Ibid.
- Dignas, Beate & Winter, Engelbert. "Rome and Persia in Late Antiquity: Neighbours and Rivals". p. 34-37, Cambridge University Press
- Gibbon, p. 829
- Gibbon, pp. 828-31
- Gibbon, p. 831, 832
- Browning 2004, p.216
- Gibbon, p. 832
- Gibbon, chap. XIII., p. 324, chap. XXIV., p. 833
- Ammianus Marcellinus, Res Gestae, 25.7.9
- Gibbon, Ibid.
- Gibbon, Edward, The Decline and Fall of the Roman Empire, The Modern Library, 1932, New York.
- Ammianus Marcellinus' works in English at the Tertullian Project with introduction on the manuscripts
- Browning, Robert, The Emperor Julian, University of California Press, 1976, ISBN 0-520-03731-6
- David S. Potter, The Roman Empire at Bay AD180-395, Routledge, 2004