The Roman emperor Julian defeats the Sasanian army in the Battle of Ctesiphon, under the walls of the Sasanian capital, but is unable to take the city.

The Roman emperor was the ruler and monarchical head of state of the Roman Empire, starting with the granting of the title augustus to Octavian in 27 BC. The term emperor is a modern convention, and did not exist as such during the Empire. When a given Roman is described as becoming emperor in English, it generally reflects his accession as augustus, and later as basileus. Another title used was imperator, originally a military honorific, and caesar, originally a cognomen. Early emperors also used the title princeps alongside other Republican titles, notably consul and pontifex maximus.

Julian was the Caesar of the West from 355 to 360 and Roman emperor from 361 to 363, as well as a notable philosopher and author in Greek. His rejection of Christianity, and his promotion of Neoplatonic Hellenism and religious tolerance in its place, caused him to be remembered as Julian the Apostate in the Christian tradition.

The Sasanian army was the primary military body of the Sasanian armed forces, serving alongside the Sasanian navy. The birth of the army dates back to the rise of Ardashir I, the founder of the Sasanian Empire, to the throne. Ardashir aimed at the revival of the Persian Empire, and to further this aim, he reformed the military by forming a standing army which was under his personal command and whose officers were separate from satraps, local princes and nobility. He restored the Achaemenid military organizations, retained the Parthian cavalry model, and employed new types of armour and siege warfare techniques. This was the beginning for a military system which served him and his successors for over 400 years, during which the Sasanian Empire was, along with the Roman Empire and later the Eastern Roman Empire, one of the two superpowers of Late Antiquity in Western Eurasia. The Sasanian army protected Eranshahr from the East against the incursions of central Asiatic nomads like the Hephthalites and Turks, while in the west it was engaged in a recurrent struggle against the Roman Empire.