The war of 337-363 (which the author dubs the 'Nisibis War'), was an exception to the traditional Roman reliance on a strategic offensive to bring about a decisive battle. Instead, the Emperor Constantius II adopted a defensive strategy and conducted a mobile defence based upon small frontier (limitanei) forces defending fortified cities, supported by limited counteroffensives by the field army of the East. These methods successfully checked Persian assaults for 24 years. However, when Julian became emperor his access to greater resources tempted him to abandon mobile defence in favour of a major invasion aimed at regime change in Persia. Although he reached the Persian capital, Ctesiphon, he failed to take it, was decisively defeated in battle and killed. The Romans subsequently resumed and refined the mobile defence, allowing Eastern provinces to survive the fall of the Western Empire.

The Nisibis War (337-363): thesis, sources, and methodology -- Background of the Nisibis War -- The military aspects of the geography -- The mid-fourth century Roman army and the strategic defense of the east -- The Persian army and the strategic offence -- Roman active defence, 337-350 -- Stalemate in Persia, 350-358 -- From the hopeless depths of misery to the height of power: the failure of Caesar Gallus -- Usurpation and crisis: campaign in the west 350-355 -- Caesar Julian: 'an emperor in strategy, a commander in tactics, a hero in combat' -- Roman passive defence 358-361 -- Roman strategic offence, 362-363 -- The march down river -- The march up country -- Conclusion.