This study of the Roman Empire’s combat with its rivals to the east examines the evolution of ancient military strategy and tactics. During the Perso-Roman wars of 337-363, Roman forces abandoned their traditional reliance on a strategic offensive to bring about a decisive victory. Instead, the Emperor Constantius II adopted a defensive strategy and conducted a mobile defense based upon small frontier forces defending fortified cities. These forces were then supported by limited counteroffensives by the Field Army of the East. These methods successfully checked Persian assaults for twenty-four years. However, when Julian became emperor, his access to greater resources tempted him to abandon mobile defense in favor of a major invasion aimed at regime change in Persia. Although he reached the Persian capital, he failed to take it. In fact, he was defeated in battle and killed. The Romans subsequently resumed and refined the mobile defense, allowing the Eastern provinces to survive the fall of the Western Empire. In this fascinating study, John Harrel applies his personal experience of military command to a strategic, operational, tactical and logistical analysis of these campaigns and battles, highlighting their long-term significance.