Defense strategy in the Roman Empire following the Crisis of the Third Century evolved considerably from the earlier preclusive security apparatus of Hadrian – which emphasized a synthesis of passive and active defense along mostly static lines of effort. Arther Ferrill credits the Emperor Constantine with the transition from the preclusive ideal to a novel defense-in-depth approach, which offered weakened frontier defenses in favor of large mobile field armies. This new model Roman army allowed more centralized control for the Emperor – as well as greater personal security – but with a vastly less controlled border region for the Empire.
“Such obvious advantages, reflecting organization of war-making capacity far beyond that of Rome’s potential opponents, gave the Roman armies a psychological edge, a superiority in morale, often sufficient in itself to deter hostile military action. In the great days of the second century, with an army of about 300,000, the Romans defended an empire of some 50,000,000 people living in the Mediterranean basin.
“More than anything else Roman grand strategy in the High Roman Empire was based on the tactical superiority of the Roman army against all potential foes. To that extent the famous walls and fortresses can be misleading. The army, not the walls or forts, defended the frontiers.
“Roman grand strategy of the second century was predicated on political stability – preclusive security requires the presence of the legions on the frontiers. Civil war and rebellion, especially when they became endemic, diverted legions from the frontiers to the interior, creating marvelous opportunities for enemies across the border. That is what happened in the third century.
“The big change in Roman grand strategy came with Constantine the Great. As Zosimus claimed in the passage quoted above, Constantine organized a large mobile field army (probably 100,000 or more), stationed centrally, by withdrawing units from the frontiers, leaving them in a weakened condition. Zosimus saw this modification of traditional Roman grand strategy as catastrophic, an interpretation endorsed by Gibbon.
“The worst feature of the defense-in-depth is that inevitably the central mobile army will become an elite force and the frontier defenders merely second rate actors in defense policy. Troops that are not expected to defeat the enemy can hardly be blamed for wanting to avoid him altogether.
*All excerpts have been taken from Fall of the Roman Empire: The Military Explanation, Thames and Hudson Inc.