Holding the Line:
Frontier Defense and the Later Roman Empire
PETER J. HEATHER
ACCORDING TO AN ANALYSIS FIRST OFFERED BY EDWARD LUTTWAK in the mid-1970s, the Roman Empire consciously moved from a frontier policy based on expansion to one based on defense in depth from the Severan era at the start of the third century AD. From this point on, its military effort was directed toward strategically planned belts of fortifications designed to absorb small-scale threats, backed by mobile, regionally based field armies held in reserve and carefully placed to deal with larger-scale incursions.1 In the summer of 370, for instance, some Saxon raiders used ships to avoid the frontier defenses of the northern Rhine and landed in northern France. Substantial raiding followed, until the local Roman commander gathered sufficient heavy cavalry and infantry units to ambush and destroy the now unsuspecting Saxons, who had been lulled into a false sense of security by a truce that ostensibly permitted them to withdraw unharmed.2 This is a textbook example of the kind of frontier strategy Luttwak identified, but on closer inspection, and despite the continuing influence of his work, which has remained solidly in print for more than thirty years, his analysis is substantially mistaken.
For one thing, while successive moments of energetic activity along the frontier are detectable in the archaeological record, some of which affected the many thousands of kilometers separating the mouth of the Rhine from that of the Danube, campaigns and fortress building can sometimes be shown to have had rather more to do with internal political agendas than with rational military planning. Keeping the