The Danube–Iller–Rhine Limes (German: Donau-Iller-Rhein-Limes) or DIRL was a large-scale defensive system of the Roman Empire that was built after the project for the Upper Germanic-Rhaetian Limes in the late 3rd century AD. In a narrower sense the term refers only to the fortifications between Lake Constance (Lacus Brigantinus) and the River Danube (Danubius);[1] in a broader sense it also includes the other Late Roman fortifications along the river Rhine (Rhenus) on the High Rhine (between Lake Constance and Basle) and on the Upper Rhine (below Basle til Bingen) as well as the Upper Danube.
The mainly Alamannic raids, especially around the middle of the 3rd century AD, necessitated a new military security plan for the northwestern borders of the Roman Empire. The Upper German-Rhaetian Limes had never been thought of as a military defensive system and was therefore abandoned after 260 (the so-called Limesfall). The frontier troops were withdrawn to positions behind the more easily controlled rivers of the Rhine ("Rhenus"), Danube ("Danuvius") and Iller ("Hilaria"). Around 290, the systematic expansion of the new military border defences began.
The defensive facilities there, as illustrated by the large number of small fortresses, were not intended to ward off major attacks, but to ensure an almost unobstructed surveillance of the limes and deter plundering. Up to 378, the Romans invariably invaded the settlements of the Germani living beyond the limes (for example, under the emperors Julian Apostata or Gratian) to punish the tribes living there and intimidate them so that they refrained from attacks on the Empire. So late-Roman frontier defence was based, on the one hand, on the fortified belt of the Danube–Iller–Rhine Limes and, on the other hand, on offensive operations and preventive strikes in the tribal areas, as well as on alliances with Germanic princes. When around 400, these punitive expeditions were discontinued, the security situation deteriorated rapidly.