Wikipedia
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, "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.