Defence - in - depth ( Roman military ) Defence - in - depth is the term used by American political analyst Edward Luttwak ( born 1942 ) to describe his theory of the defensive strategy employed by the Late Roman army in the 3rd and 4th centuries AD . Luttwak ' s " Grand Strategy of the Roman Empire " ( 1976 ) launched the thesis that in the 3rd and early 4th centuries , the Imperial Roman army ' s defence strategy mutated from " forward defence " ( or " preclusive defence " ) during the Principate era ( 30 BC - AD 284 ) to " defence - in - depth " in the 4th century . " Forward - " or " preclusive " defence aimed to neutralise external threats before they breached the Roman borders : the barbarian regions neighbouring the borders were envisaged as the theatres of operations . In contrast , " defence - in - depth " would not attempt to prevent incursions into Roman territory , but aimed to neutralise them on Roman soil - in effect turning border provinces into combat - zones . Scholarly opinion generally accepts " forward - defence " as a valid description of the Empire ' s defensive posture during the Principate . But many specialists in Roman military history ( which Luttwak is not ) contest that this posture changed to Luttwak ' s " defence - in - depth " from 284 onwards . Described as " manifestly wrong " by the expert on Roman borders C . R . Whittaker , " defence - in - depth " has been criticised as incompatible with 4th - century Roman imperialist ideology ( which remained expansionist ) , Roman strategic planning capabilities , and with the evidence of 4th - century Roman historian Ammianus Marcellinus and with the vast corpus of excavation evidence from the Roman border regions . Thesis . According to this view , the Imperial Roman army had relied on neutralizing imminent barbarian incursions before they reached the imperial borders . This was achieved by stationing units ( both legions and auxilia ) right on the border and establishing and garrisoning strategic salients beyond the borders ( such as the Agri Decumates in SW Germany ) . The response to any threat would thus be a pincer movement into barbarian territory : large infantry and cavalry forces from the border bases would immediately cross the border to intercept the coalescing enemy army ; simultaneously the enemy would be attacked from behind by crack Roman cavalry ( " alae " ) advancing from the strategic salient ( s ) . This system obviously required first - rate intelligence of events in the barbarian borderlands , which was provided by a system of watch towers in the strategic salients and by continuous cross - border scouting operations ( " explorationes " ) . According to Luttwak , the forward defence system was always vulnerable to unusually large barbarian concentrations of forces , as the Roman army was too thinly spread along the enormous borders to deal with such threats . In addition , the lack of any reserves to the rear of the border entailed that a barbarian force that successfully penetrated the perimeter defenses would have unchallenged ability to rampage deep into the empire before Roman reinforcements could arrive to intercept them . The first major challenge to forward defense was the great invasion of Germanic tribes ( esp . Quadi and Marcomanni ) across the Danube in 166 - 7 , which began the Marcomannic Wars . The barbarians reached as far as Aquileia in northeastern Italy and were not finally expelled from the empire until 175 . But the response of the imperial high command was not to change the forward defence strategy , but to reinforce it ( by the founding of 2 new legions under Marcus Aurelius and 3 more under Septimius Severus and probably matching auxiliary forces ) . It was only after the catastrophic military crises of 251 - 71 that the Roman command under Diocletian turned to defence - in - depth : but only out of necessity , not conviction , as there were attempts to return to forward defence as late as Valentinian I ( ruled 364 - 75 ) Forward defence had become simply too costly to maintain , especially with the emergence of a more powerful and expansionist Persian empire ( the Sassanids ) which required greater deployments in the East . The essential feature of defence - in - depth , according to Luttwak , was the acceptance that the Roman frontier provinces themselves would become the main combat zone in operations against barbarian threats , rather than the barbarian lands across the border . Under this strategy , border forces would not attempt to repel a large incursion . Instead , they would retreat into fortified strongholds and wait for mobile forces ( " comitatenses " ) to arrive and intercept the invaders . Border forces would be substantially weaker than under forward defence , but their reduction in numbers ( and quality ) would be compensated by the establishment of much stronger fortifications to protect themselves : hence the abandonment of the old " playing - card " rectangular design of Roman fort . The new forts were so designed that they could only be taken with the use of siege engines ( which barbarians generally lacked ) : square or even circular layout , much higher and thicker walls , wider perimeter berms and deeper ditches ; projecting towers to allow enfilading fire ; and location in more defensible points , such as hilltops . At the same time , many more small forts were established in the hinterland , especially along roads , to impose delays on the invaders . Also , fortified granaries were built to store food safely and deny supplies to the invaders . Finally , the civilian population of the province was protected by providing walls for all towns , many villages and even some villas ( large country houses ) ; some pre - Roman hillforts , long since abandoned , were re - occupied in the form of new Roman walled settlements . The invading force would thus find itself in a region peppered with strongholds in enemy hands and where it could not easily get access to sufficient supplies . If the invaders ignored the strongholds and advanced , they risked sorties and attacks in the rear . If they attempted to besiege the strongholds , they would give the mobile troops valuable time to arrive . Overall , the aim of defence - in - depth was to provide an effective defence system at a sustainable cost , since defence - in - depth required much lower troop deployments than forward defence . To be more precise , the cost was transferred from general taxpayers to the people of the frontier provinces , especially the rural peasantry , who , for all the fortifications , would often see their family members killed or abducted , homes destroyed , livestock seized and crops burnt by marauding barbarians . Critique . Luttwak ' s work has been praised for its lucid analysis of , and insights into , issues regarding Roman military dispositions , and for stimulating much scholarly debate about these issues . But the validity of his basic thesis has been strongly disputed by a number of scholars , especially in a powerful critique by B . Isaac , the author of the fundamental study of the Roman army in the East ( 1992 ) . The objections fall under two broad headings : ( 1 ) The Roman empire did not have the intelligence and planning capacity to sustain a " grand strategy " and in any case was not defensive in ideology or policy . ( 2 ) Defence - in - depth is not , in the main , consistent with the literary and archaeological evidence . Strategic planning . Luttwak ' s thesis of an imperial grand strategy rests on a number of assumptions : ( a ) that the empire ' s strategic posture was basically defensive ; ( b ) that Roman expansion and choice of borders were systematic and rational , with the main objective of securing defensible borders ; ( c ) that the Roman government ' s primary concern was to ensure the security of its provincial subjects . But Isaac demonstrates that these assumptions are probably false and result from inappropriate application of modern concepts of international relations and military strategy to the ancient world . Isaac suggests that the empire was fundamentally aggressive both in ideology and military posture , up to and including the 4th century . This was demonstrated by the continued military operations and siting of fortifications well beyond the imperial borders . The empire ' s expansion was determined mostly by the ambitions of emperors ; and that the choice of borders , to the extent that they were planned at all , was more influenced by logistical considerations ( e . g . rivers , that were critical conduits for supplies ) rather than defensibility . Finally , the imperial government probably was far less concerned with the security of its subjects than would be a modern government . Isaac shows that the empire did not develop the centralised military planning , or even accurate enough cartography , necessary to support grand strategy . Rome did not develop the equivalent of the centralised general staff of a modern army ( and even less strategic studies institutes of the kind frequented by Luttwak ) . Emperors depended on the theatre military commanders ( the provincial governors , later the " magistri militum " and " duces " ) for all military intelligence . Archaeological evidence . There is also little unequivocal archaeological and literary evidence to support defence - in - depth . Luttwak ' s defence - in - depth hypothesis appears to rely on two basic features : ( a ) deepened fortified border zones : " It became necessary to build forts capable of sustained resistance , and these fortifications had to be built in depth , in order to protect internal lines of communication . Instead of a thin perimeter line on the edges of provincial territory , broad zones of military control had to be created . . . " " The thin line of auxiliary ' forts ' and legionary ' fortresses ' was gradually replaced by a much broader network of small fortified hard - points ( in the hands of ) scattered groups of static " limitanei " . . . " The hypothesis thus predicts the establishment of fortifications well into the interior of border provinces , rather than just a string of bases right on the border line ; ( b ) the use of the " comitatus praesentales " ( imperial escort armies ) as interception forces to deal with incursions . In this regard , it is to be noted that Luttwak terminates his analysis in 350 , before the establishment of the regional " comitatus " . The interception forces were thus the single large " comitatus " of Constantine , and , later , the 3 " comitatus " known from Ammianus to exist in 350 of Gaul , Illyricum and the East . But there are serious difficulties with both propositions . ( a ) J . C . Mann points out that there is no evidence , either in the " Notitia Dignitatum " or in the archaeological record , that units along the Rhine or Danube were stationed in the border hinterlands . On the contrary , virtually all forts identified as built or occupied in the 4th century on the Danube lay on , very near or even beyond the river , strikingly similar to the 2nd - century distribution . Luttwak seizes on the situation in Palaestina Salutaris ( mainly the former Arabia Petraea ) province , which was dotted with forts all over , as an example of defence - in - depth . But here it cannot be proven that the defence system developed only in the 4th century . It may have dated from as early as the 2nd century . In any case , Isaac shows that these " in - depth " forts were probably used for the purposes of internal security against rebels and brigands rather than defence against external threat . Indeed , such material as can be dated to Diocletian suggests that his reorganisation resulted in a massive reinforcement of linear defence along his newly built desert highway , the " Strata Diocletiana " . In Britain , the configuration of a large number of 4th - century units stationed between Hadrian ' s Wall and the legionary fortresses at " Deva " ( Chester ) and " Eboracum " ( York ) , superficially resembles defence - in - depth . But the same configuration existed in the 2nd century , and was due to the short length of the frontier , forcing a " vertical " rather than horizontal deployment , as well as the need to protect the coastlines from seaborne attack . It was not defence - in - depth in the Luttwak sense . So strong is the evidence for forward defence under Diocletian that Luttwak himself struggles to avoid that conclusion . At one point , he describes it as " shallow defence - in - depth " , a contradiction in terms . At another , he admits that Diocletian ' s policy was a " sustained attempt to provide a preclusive ( i . e . forward ) defence of the imperial territory " . Indeed , the unfavourable evidence forces Luttwak to adopt a self - contradicting thesis . While claiming that the basic strategy of the 4th century was defence - in - depth , he admits that there were repeated attempts by the stronger emperors ( up to and including Valentinian I ) to revert to forward defence . This obviously casts doubt on whether a defence - in - depth strategy was ever contemplated or implemented in reality . The Romans continued to assist the client tribes to defend themselves in the 4th century e . g . the construction by Constantine ' s army of two massive lines of defensive earthworks ( the Devil ' s Dykes in Hungary and the Brazda lui Novac de Nord in Romania ) well beyond the Danube ( 100 – 200 miles forward ) to protect the client tribes of the Banat and the Wallachia n plain against Gothic incursions . This system of a series of buffer zones of " client tribes " clearly represents an efficient and economical form of " forward defence " . It contradicts the proposition that the border provinces of the empire were themselves envisaged as buffer zones . Literary evidence . In the absence of any evidence of " defensive depth " in the stationing of border forces , the only " depth " left were the " comitatus praesentales " ( imperial escort armies ) stationed in the interior of the empire . But Luttwak himself admits that these 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 they are often described as " mobile field armies " , in this context " immobile " would be a more accurate description . Luttwak terminates his analysis in mid - 4th century , just before the establishment of the regional " comitatus " . But the positioning of the latter , right on the borders or within 60 miles ( 100 km ) of them , seems strikingly similar to that of the legions in the 2nd century . It could be argued that the deployment of the regional " comitatus " was simply an admission that Zosimus ' criticism of Constantine ' s policy was valid and that effective forward defence required reinforcement of the " limitanei " troops . A further powerful objection to defence - in - depth is that it is clear from Ammianus that Rome continued major offensive operations across the imperial borders in the 4th century . These were strikingly similar to the pincer movements described by Luttwak as being characteristic of forward defence in the early Principate . For example , Valentinian I ' s campaign against the Quadi in 375 . The barbarian tribe that were the target of the operation rarely resisted the Romans in pitched battle and more often took refuge in forests and hills . The Romans would then systematically ravage their crops and burn their hamlets until starvation forced the barbarians to surrender . They would then be forced to conclude treaties of alliance with the Romans , often involving the client status described below . But there was no aspect of this activity that was peculiar to the 4th century . One " defence strategy " the empire certainly employed was a system of treaties of mutual assistance with tribes living on the imperial frontiers , but this was not unique to the 4th century , but a long - standing practice dating to the days of the late Republic . 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 . In many cases , the loyalty of the ally would need to be further secured by gifts or regular subsidies . In some cases , the Romans would assume a loose suzerainty over the tribe , in effect dictating the choice of new chiefs . This practice was applied on all the frontiers : Germans along the Rhine , Sarmatians along the Danube , Armenian kings and Caucasian and Saracen tribes on the Eastern frontier and Mauri in North Africa . On the desert frontier of Syria , the Romans would appoint a Saracen sheikh ( called a " phylarch os " in Greek ) , according him an official rank in the Roman hierarchy , to " shadow " each " dux limitis " in the sector . In return for food subsidies , the phylarchs would defend the desert frontier against raiders . State of the debate . As regards imperial ideology and central defence planning , Adrian Goldsworthy argues that both sides of the debate , which continues vigorously , have made valid points . Some degree of central planning is implied by the disposition , frequently altered , of legions and auxiliary forces in the various provinces . In addition , although the empire ' s ideology may have been offensive in nature , border fortification such as Hadrian ' s Wall was clearly defensive . It is a fact that the empire ceased to expand its territory after the rule of emperor Trajan ( 98 - 117 ) . Thereafter , the borders remained largely static , with indeed a few losses of territory : the immediate evacuation of Trajan ' s conquests in Mesopotamia by his successor Hadrian ( r . 117 - 38 ) and of the Agri Decumates in Germany and of Dacia in the 3rd century . Thus , even if the empire ' s ideology and propaganda was expansionist ( the slogan " imperium sine fine " - " empire without limits " - was common ) , its policy was in reality generally non - expansionist . As regards Luttwak ' s defence - in - depth theory itself , there appears to be insufficient clearcut evidence to support it and massive evidence against it . Mann ' s critique was written in 1979 , so does not take account of the substantial corpus of archaeological data accumulated since . But the latter overwhelmingly contradicts a defence - in - depth strategy . Virtually all identified forts built in the 4th century lay on , very near or even beyond the border . Some evidence of fortifications in the hinterland has come to light that could be consistent with defence - in - depth . But such features cannot be unequivocally linked with military units . Furthermore , the 4th - century army ' s " defence " posture shares many features with the earlier forward - defence policy . The undoubted enhanced fortification of forts and other buildings , as well as cities in the border provinces ( and deep in the interior of the empire including Rome itself ) may therefore be interpreted as simply an admission that forward defence was not working as well as in the earlier centuries . Either barbarian pressure was much greater and / or the Roman border forces were less effective than before in containing it .