The Emperor Julian’s army with his Armenian allies invaded the Sassanian Empire in 363 A.D. This was the last major invasion of the Persian Empire by Rome. Julian prepared meticulously for the invasion of the Sassanian realms. He re-organized the legions of the East and turned them into formidable fighting units. Julian also brought with him experienced legionnaires from the west. Julian establish a flotilla of ships to carry his legions deep into Persian territory and to supply his forces<ref> Tougher, Shaun. Julian the Apostate. Edinburg, Edinburgh University Press. 2007), p. 27 </ref>. This was an innovative strategy. Julian conducted the early stages of the invasion very impressively. He had asked his ally Armenia to concentrate a large force on their mountainous border with Persia. This had convinced many in Persia that Rome would invade via the highlands of Armenia. Many previous Roman armies had followed this route. In this terrain, the Roman legionnaires had the advantage over the formidable Persian cavalry, especially the famed horse archers. In a daring move, Julian invaded through the Syrian desert and followed the route of the Euphrates River. The Persian Shah Shapur II had been deceived by Julian and was unprepared for the invasion. Julian dispatched a decoy force under Procopius into Persian territory to further confuse Shapur II and his court. Julian’s forces advanced rapidly, and he forced the Sassanian king to retreat. The transport fleet allowed a large body of troops to be provisioned. Julian feigned a march towards the Tigris, hoping to draw into a battle. At this time, he divided his forces and a large army under Procopius invaded Mesopotamia. Upon reaching Carrhae, the main Roman force under Julian turned south and rejoined the transport fleet to begin the main invasion and its goal was to capture of the Sasanian winter capital of Ctesiphon <ref> Tougher, p. 113</ref>. If Julian captured the city, he would have shown that paganism had restored Rome to greatness. It would also force the Sassanian Shah to the east and ensure that he could not attack the Roman frontier. Julian defeated a large Persian army on the outskirts of the city of Ctesiphon and he besieged it. However, he failed to take the city and quietly abandoned the siege and he burned his flotilla of ships and marched to meet up with the army of Procopius. Many argue that he should have retreated to Syria and abandoned Procopius, but Julian was an honorable man. He joined up with the other army and they made their way north to reach Armenia. The Persians constantly harried the Romans and inflicted heavy casualties <ref>Murdoch, Adrian, The Last Pagan (UK: Sutton Publishing Limited, 2003), p. 113 </ref>. It appeared that Julian’s army the largest possibly ever assembled, for an invasion of Persia was about to be destroyed. The two armies clashed at Samarra now in Northern Iraq, but the battle was inconclusive. However, Julian was wounded, and he died three days later. His successor Jovian saved the army with a dishonorable peace, and the new Emperor had to pay tribute to the Persians. Julian’s polices that favored paganism were quietly abandoned and within decades paganism was outlawed by Emperor Theodosius the Great.
At first it appeared that Julian was waging a brilliant campaign and he was undoubtedly a talented strategist. Julian failed to appreciate the level of resistance as he advanced on Ctesiphon. This was a critical mistake and resulted in Julian losing the initiative. The resistance on the route to Ctesiphon allowed the Persian Shah, Shapur II to gather his forces and to assemble a large army. The other great mistake was that Julian had divided his forces, a basic and unforgivable error. He ordered Procopius to advance into Mesopotamia, but this achieved little and only weakened his own army. After the failure to take the winter-capital of the Persians, Julian instead of retreating to safety in Syria had to rescue the army of Procopius <ref>Murdoch, p. 118</ref>. If Procopius forces had been with the main force, Julian could have forced a retreated down the Euphrates with his fleet of ships in support. Instead the Emperor was obliged to move towards the Tigris, right into the heart of Persia. It was during this effort to contact Procopius that the Battle of Samarra took place. In this Julian was wounded and he later died of his wounds. This ultimately led to his army becoming stranded in Persian territory. Furthermore, Julian did not have the technical capability to engage in a lengthy siege of the Sassanian capital. There were very few siege engines with the army, such as a battering ram or a ballista. Ammianus, in his histories asserts that Julian hoped that an overwhelming show of force would persuade Ctesiphon to surrender<ref> Ammianus, vi, i</ref
?. This was a mistake and the Persians defended the walls of their city, fiercely. This was another misjudgment of Julian and typical of the wishful thinking that characterized the entire campaign.
[[File: Julian 4.jpg|200px|thumb|left|ruins of Ctesiphon from a 19th century drawing]]