century on the Danube lay on, very near or even beyond the river, strikingly similar to the 2nd-century distribution. Another supposed element of "defence-in-depth" were the comitatus praesentales (imperial escort-armies) stationed in the interior of the empire. A traditional view is that the escort-armies' role was precisely as a strategic reserve of last resort that could intercept really large barbarian invasions that succeeded in penetrating deep into the empire (such as the invasions of the late 3rd century). But these large comitatus were not established before 312, by which time there had not been a successful barbarian invasion for c. 40 years. Also Luttwak himself admits that they were too distant from the frontier to be of much value in intercepting barbarian incursions. Their arrival in theatre could take weeks, if not months. Although the comitatus praesentales are often described as "mobile field-armies", in this context "immobile" would be a more accurate description. Hence the mainstream modern view that the central role of comitatus praesentales was to provide emperors with insurance against usurpers. Luttwak terminates his analysis at the end of Constantine's reign, before the establishment of the diocesan comitatus. Unlike the imperial escort-armies, these were close enough to the theatre of operations to succour the border troops. But their stationing may have differed little from the location of legions in the 2nd century, even though they apparently wintered inside cities, rather than in purpose-built legionary bases. For example, the two comitatus of Illyricum (East and West) are documented as wintering in Sirmium, which was the site of a major legionary base in the Principate. Furthermore, the late empire maintained a central feature of the forward defence of the Principate: a system of treaties of mutual assistance with tribes living on the imperial frontiers. The Romans would promise to defend the ally from attack by its neighbours. In return, the ally would promise to refrain from raiding imperial territory, and prevent neighbouring tribes from doing the same. Although the allies would officially be denoted tributarii (i.e. subject to paying tribute to Rome, in cash or in kind), in practice the loyalty of the ally was often secured by gifts or regular subsidies from Rome. This practice was applied on all the frontiers. The Romans continued to assist the client tribes to defend themselves in the 4th century. For example, Constantine I's army constructed two massive lines of defensive earthworks, 100–250