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 third and fourth centuries AD.

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 meant 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.

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).

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.

To be more precise, the cost was transferred from general taxpayers to the people of the frontier provinces,[7] 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.

[8] 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).

[9] 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.

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.

[13] 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.

[11] 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.

[19] On the contrary, virtually all forts identified as built or occupied in the fourth century on the Danube lay on, very near or even beyond the river, strikingly similar to the second-century distribution.

[23] 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 fourth-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 second 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.

While claiming that the basic strategy of the fourth 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.

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.

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 fourth century, but a long-standing practice dating to the days of the late Republic.

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 third century.

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.