A study of the role of riverine shipping in Late Roman defensive strategy along the Danube (300-600 AD). This work follows and draws together current trends amongst ancient historians who take a more positive view than hitherto of the value of rivers as natural barriers and of the defensive qualities of late-Roman frontier units. The Danube frontier, which was more than 2,500 kilometers long, simultaneously presented both advantages and disadvantages to its defenders. The formation of a large number of well-equipped naval units tasked with monitoring and protecting the river banks moved the first line of defence forward from the region immediately behind the rivers onto the rivers themselves. These units also compensated, at least partially, for the weakening of frontier defences brought about by the new strategy of Diocletian and especially that of Constantine, which located comitatenses troops in the hinterland. The general view of a late-Roman defensive system consisting simply of limitatenses units stationed in the immediate riparian area with field-army units based at road junctions further behind the frontiers in fact ignores more than a dozen naval units established in the late 3rd and 4th centuries.