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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 Roman 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,〔Whittaker (1994) 〕 "defence-in-depth" has been criticised as incompatible with 4th-century Roman imperialist ideology (which remained expansionist), Roman strategic planning capabilities, 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).〔Luttwak (1976) Fig.3.3〕 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.〔Luttwak (1976) 136〕 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).〔Luttwak (1976) 131-2〕 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)〔Luttwak (1976) 132〕 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.〔Luttwak (1976) Fig.3.2〕 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,〔Luttwak (1976) 137〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Defence-in-depth (Roman military)」の詳細全文を読む スポンサード リンク
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