Julian's Persian expedition took place from March to July 363 AD. It was the final military campaign to be undertaken by Roman emperor Julian, who died in battle at Samarra. The Roman Empire fought against the Sasanian Empire, which was ruled at the time by Shapur II, and sought to capture the Sasanian capital city Ctesiphon.
In order to mislead the enemy and to carry out a pincer attack, Julian sent a detachment to join with his ally Arshak II of Arsacid Armenia to take the Tigris route from the north. Meanwhile, his main army and a large fleet, facing little opposition, advanced rapidly down the Euphrates, destroying several well-fortified cities along the way. The ships then reached the walls of Ctesiphon, where a large Sasanian army had assembled in defensive positions.
The Romans won a victory outside the city, but the northern army failed to arrive, and the fortified capital seemed impregnable. Due to these complications, Julian did not attempt to besiege the city, instead burning the Roman fleet of supplies and leading a march into Persia. Shapur II's army, however, took lengths to avoid a full-scale battle and used a scorched-earth strategy, which eventually left Julian's army facing supply problems and forcing their retreat to the north. Shapur II's army pursued the retreating Romans, ceaselessly harassing the retreat through skirmishes. Julian died of wounds sustained from one of these skirmishes and was succeeded by Jovian, who agreed to surrender to the Sasanians under unfavourable terms in order to save the remnants of his demoralized and exhausted army from annihilation.
The Peace Treaty of 363 transferred multiple regions and frontier fortresses, including Nisibis and Singara, to the Sasanians. In addition to making territorial concessions, the Roman Empire renounced its alliance with Armenia, giving Shapur II the opportunity to invade and annex it.
Aims and preparations
The military and political aims of the campaign are uncertain, and they are also disputed by both ancient and modern sources and historians. According to Ammianus Marcellinus, Julian's aim was to enhance his fame as a general and to punish the Persians for their invasions of Rome's eastern provinces; for this reason, he refused Shapur's immediate offer of negotiations. Julian was a devout believer in the old Roman religion. Some modern authors note that he intended to accelerate and gain support for the pagan renovation of the Roman Empire and actions against the Christians after defeating the Sasanian Empire, since such victory would have been proof of the support of the Roman gods. Among the leaders of the expedition was Hormizd, a brother of Shapur II, who had fled from the Persian Empire forty years earlier and had been welcomed by the then Roman emperor Constantine I. Julian is said to have intended to place Hormizd on the Persian throne in place of Shapur, but replacing a successful ruler with one who has been in exile for decades is an "incomprehensible" aim.
Julian asked several major oracles about the outcome of his expedition. Julian's praetorian prefect of Gaul, Sallustius, wrote advising him to abandon his plan, and numerous adverse omens were reported; at the urging of other advisers he went ahead. He instructed Arshak II of Armenia to prepare a large army, but without revealing its purpose; he sent Lucillianus to Samosata in the upper Euphrates valley to build a fleet of river ships. These preparations are thought by scholars to have suggested to Shapur that an invasion from the north, by way of the Tigris valley, was Julian's plan.
Advance
Julian had wintered at Antioch in Roman Syria. On 5 March 363, he set out north-east with his army by way of Beroea (Aleppo) and Hierapolis (Manbij), where fifty soldiers were killed when a portico collapsed while they were marching under it. The whole army mustered there, crossed the middle Euphrates and proceeded to Carrhae (Harran), the site of the famous battle in which the Roman general Crassus was defeated and killed in 53 BC. "From there two different royal highways lead to Persia," writes the eyewitness Ammianus Marcellinus: "the one on the left through Adiabene and across the Tigris; the one on the right through Assyria and across the Euphrates." Julian made use of both. He sent a detachment, which ancient sources variously estimated between 16,000 and 30,000 in strength, under Procopius and Sebastianus towards the Tigris where they were to join Arshak and his Armenian army. They were then to attack the Persians from the north. Thus, by tying Shapur down in northern Mesopotamia, Julian would have been able to quickly advance down the Euphrates without opposition, while the group in Armenia was supposed to join up with Julian in Assyria. Many modern scholars have praised the choice of routes, rapid movements, and deception, while some consider the plan to be inadequate with regard to supply, communication, climate consideration, and the difficulty of crossing between Euphrates and Tigris near Naarmalcha.
Julian himself, with the larger part of his army of 65,000, of which it is unclear whether that was before or after Procopius' departure, turned south along the Balikh River towards the lower Euphrates, reaching Callinicum (al-Raqqah) on 27 March and meeting the fleet of 1,100 supply vessels and 50 armed galleys under the command of Lucillianus. There he was met by leaders of the "Saraceni" (Arab nomads), who offered Julian a gold crown. He refused to pay the traditional tribute in return; though he later requested their envoys join him. The army followed the Euphrates downstream to the border city of Circesium and crossed the river Aboras (Khabur) with the help of a pontoon bridge constructed for the purpose.
Progress of the war
From Circesium to Ctesiphon
Once over the border, Julian invigorated the soldiers' ardor with a fiery oration, representing his hopes and reasons for the war, and distributed a donative of 130 pieces of silver to each. The army was divided on the march into three principal divisions. The center under Victor, composed of the heavy infantry; the cavalry under Arintheus and Hormizd the renegade Persian on the left; the right, marching along the riverbank and maintaining contact with the fleet, consisting likewise of infantry, and commanded by Nevitta. The baggage and the rearguard were under Dagalaifus, while the scouts were led by Lucilianus, the veteran of Nisibis. A detachment was left to hold the fortress of Circesium, as several of the fickle Arabian tribes near the border were allied with Persia.
Julian then penetrated rapidly into Assyria. Similarly to the concentration of Egypt's population on the Nile was the main part of the population of Assyria located in the towns on the banks of the Euphrates, while the interior of the country was, for the most part, a desert wasteland. The first Persian settlement that was encountered was Anatha (Anah), which surrendered, but the Romans destroyed it. The army then moved past the Thilutha (Telbis Island, now flooded by the Haditha Dam) and Achaiachala (variously identified with Haditha or Bijân Island, the latter is now flooded by the Haditha Dam), since both of them were difficult to capture. A part of the army crossed over at Baraxmalcha. Then they reached Diacira (Hit) and then Ozogardana/Zaragardia, both of which were abandoned and Julian destroyed them. Then the Romans met a Sasanian detachment for the first time and defeated it. After Macepracta/Besechana, which was reached by a march of two weeks, Julian's army besieged Pirisabora (Anbar), the largest city of Mesopotamia after Ctesiphon. The city was surrendered after two or three days and was destroyed. Julian used the nearby Naarmalcha (the Royal Canal), which was the most direct route, to transfer the fleet from the Euphrates to Tigris. The army moved southeast with great difficulties and losses. Worse still, the Persians flooded the land by destroying the dams and diverting the water of the Naarmalcha canal after the Romans passed Phissenia. Julian's army then reached Bithra. Arriving within a dozen miles of Ctesiphon, the fortified Maiozamalcha was besieged and taken after several days through mining operations. The city was destroyed and almost all inhabitants were killed. Through Meinas Sabatha, Julian marched towards the Seleucia–Ctesiphon metropolis (Al-Mada'in).
Arshak II's preparations in Armenia might have deceived Shapur II, but he must have been soon informed of the advance of the main Roman army along the Euphrates. Modern scholars have noted this avoidance of a full-scale field battle and letting the main Roman army advance deep into Assyria.
Ctesiphon
After destroying the private residence, palaces, gardens and extensive menagerie of the Persian monarchy north of Ctesiphon, and securing his position by improvised fortifications, Julian turned his attention to the city itself.
The twin cities of Ctesiphon and Seleucia (rebuilt as Veh-Ardashir) lay before Julian to the south. In order to invest the place on both sides, Julian first dug a canal between the Euphrates and Tigris, allowing his fleet to enter the latter river, and by this means ferried his army to the further bank. A large Persian army had assembled in Ctesiphon, which was the appointed place of rendezvous for Shapur's army at the outset of the campaign; this was arrayed along the eastern bank in strong defensive positions, and it required the advantages of night-time and surprise, and subsequently a prolonged struggle on the escarpment, reportedly lasting twelve hours, to gain the passage of the river. But in the contest victory lay ultimately with the Romans, and the Persians were driven back within the city walls after sustaining losses of twenty-five hundred men; Julian's casualties are given at no more than 70.
Though Julian had brought with him through Assyria a large train of siege engines and offensive weapons, and he was supplied by an active fleet that possessed the undisputed navigation of the river, the Romans appear to have been at some difficulty in putting Ctesiphon to the siege. Although it had fallen on several previous occasions to the Romans, the city was better fortified than in the second century.
Confronted with the difficulty to capture the city, Julian called a council of war, at which it was decided not to besiege the city and march into Persia's interior instead—a turning point of the campaign. Apparently, it met with resistance within the army. The reasons for this decision are uncertain; according to Ammianus, it was due to the fear of a two-front war, since Shapur II's army was apparently nearby. According to Libanios, Shapur II sent an emissary to Julian, who refused it. After abandoning the siege, Julian burned his fleet of provisions to avoid its fall into the Sasanians' hands, as Julian's army now had to move upstream. Another theory is that Julian was deceived by Sasanian deserters. Julian was possibly still hoping to join up with the reinforcements from Armenia and then defeat Shapur II's army in a regular battle.
It is possible the intention was justified by the hope of destroying the army of Shapur before the latter should join with the already numerous garrison of Ctesiphon to besiege the camp of the besiegers. More inexplicable is the burning of the fleet, and most of the provisions, which had been transported the whole course of the Euphrates with such monumental cost. Although ancient and modern historians have censured the rashness of the deed, Edward Gibbon palliates the folly by observing that Julian expected a plentiful supply from the harvests of the fertile territory by which he was to march, and, with regard to the fleet, that it was not navigable up the river, and must be taken by the Persians if abandoned intact. Meanwhile, if he retreated northward with the entire army immediately, his already considerable achievements would be undone, and his prestige irreparably damaged, as one who had obtained success by stratagem and fled upon the resurgence of the foe. There were therefore no negligible reasons for his abandonment of the siege, the fleet, and the safe familiarity of the river bank.
Ctesiphon to Samarra
After spending several days outside Ctesiphon, Julian directed his army toward the inner regions of Persia east of Ctesiphon. Shapur II's army avoided being dragged into battle while following a scorched earth policy by firing houses, provisions, crops and farmland wherever Julian's march approached; since the army had preserved only 20 days' provisions from the ruin of the fleet, they soon faced the threat of starvation. This together with summer heat prompted the Romans not to advance further and instead seek a route towards the Tigris and follow it upstream to the region Corduena.
At this stage, Shapur II's army appeared and began to engage Julian's army by ceaseless skirmishing. The Sasanian cavalry repeatedly assailed the Romans' extended columns in the retreat; at Maranga a sharp skirmish developed into a battle; the Sasanians were repulsed, and Julian's army retired to rest in the hills south of Samarra, on July 25, 363.