Placing Edward Luttwak’s Grand Strategy of the Roman Empire (Baltimore: The Johns Hopkins University Press, 1976) in the category of a classic is bound to generate controversy, as did Luttwak’s book when it was first published in 1976 (it was his doctoral dissertation at Johns Hopkins University SAIS). Some prominent scholars of Rome and the classical period dismissed the book as the product of an index-flipping amateur. They brought up particular inaccuracies and challenged many of its generalizations and interpretations. In the broader public policy arena, in the aftermath of Vietnam and the period of detente with the Soviet Union, those who wanted to pare down America’s global role did not take well to sympathetic studies of imperial strategy, grand or otherwise. On the other hand, for those who took strategy seriously and who fretted over the decline of American power and will, Luttwak’s emphasis on suasion and the economy of force too often seemed to offer a solution set that was too good to be true.
That said, Luttwak was clearly on to something. He put forward a challenging perspective on grand strategy, harkening in some ways to the realist perspective of men like Walter Lippmann, but also full of fresh insight. He raised the right sort of questions – new questions – even if he did not necessarily reach the right or only answer. He did so by outlining what he perceived to be the patterns of continuity and change in the strategic policies of the Roman Empire from Augustus to Diocletian. He concluded that the ancient Romans had an instinctive grasp of strategic logic embedded in their culture, which allowed not only continuity across changing regimes, but also for strategic evolution in response to a changing security environment.
Luttwak posed the central problem that grand strategy, in his view, properly seeks to address: “For the Romans, as for ourselves, the two essential requirements of an evolving civilization were a sound material base and adequate security. For the Romans, as for ourselves, the elusive goal of strategic statecraft was to provide security without prejudicing the vitality of its economy base and without compromising the stability of an evolving political order.” Rome’s enduring success, in Luttwak’s opinion, reflected the high degree to which these conflicting imperatives were reconciled at the highest level of statecraft. “Roman security was not derived from tactical superiority on the battlefield, from superior generalship, or from a more advanced weapons technology. The superiority of the Empire was of an altogether more subtle order: it derived from the whole complex of ideas and traditions that informed the organization of Roman military force and harnessed the armed power of the empire to political purposes. The firm subordination of tactical priorities, martial ideas, and warlike instincts to political goals was the essential condition of the strategic success of the empire.” The Romans, with rare exceptions, avoided the costly pursuit of purely tactical successes and purposeless victories. Especially in the early period of the Empire, military force was clearly recognized for what it was, an essentially limited instrument of power, costly and brittle.
The Romans understood that, when possible, it was best to conserve force and use military power indirectly as the instrument of political warfare. Together with money and manipulative diplomacy, the Romans deployed forces visibly ready to fight but held back from battle to foster disunity among those who might jointly threaten the empire, to deter those who would otherwise attack, and to control lands and peoples by intimidation – ideally to the point where sufficient security or even an effective domination could be achieved without any use of force at all. The Romans learned that most desirable use of military power was not military at all, but political. They conquered the entire Hellenistic world with few battles and much coercive diplomacy. The Romans understood all the subtleties of deterrence, and its limitations.
Above all, the Romans clearly realized that the dominant dimension of power was not physical but psychological — the product of others’ perceptions of Roman strength, rather than the use of this strength. The Roman siege of Masada, an otherwise unimportant place, was a calculated act of psychological warfare that revealed the exceedingly subtle workings of a long range security policy based on deterrence. The Romans did not bypass the fortress and starve out the Jews using a small blockading force, a course which would have attracted little attention. Nor did they storm the mountain, with the attendant loss of manpower and the risk of at least a temporary setback that might encourage resistance elsewhere. The full-scale siege operation was not cheap in terms of time or treasure, to be sure. But by engaging in a three-year operation to reduce the fortress by great works of engineering, the Romans must have made a great impression on those who might otherwise been tempted to contemplate revolt.
Luttwak identified three distinct systems of imperial security that integrated diplomacy, military forces, road networks, and fortifications, which were intended to serve a single objective. The design of each element reflected the logic of the whole. Each system was intended to satisfy a distinct set of priorities, themselves the reflection of changing conceptions of empire: hegemonic expansion for the first system; territorial security for the second, and finally, in diminished circumstances, sheer survival for the imperial power itself. Each system was based on a different combination of diplomacy, direct force, and fixed infrastructures. Each entailed different operational methods.
The system of what might be called the Republican Empire, or Julio-Claudian, derived its security along the frontier from client states and mobile armies. Around its core areas the empire was hegemonic in nature, with client states autonomously responsible for implementing the Roman desiderata and providing for the territorial security of the core areas out of their own resources. No Roman troops were ordinarily deployed in the client states or with the client tribes, but the stability of the system required a constant diplomatic effort both to ensure that each client was continually aware of the totality of Roman power (while being politically isolated from each other) and to maintain the internal and regional (i.e., inter-client) equilibrium of the client structure. Client states great and small were thus kept in subjection by their own perceptions of Roman power, and this deterrent force was complemented by positive inducements, notably subsidies.
Under this system, the Roman Army that the clients perceived as an undivided force of overwhelming strength was actually distributed in a vast circle around Rome. But these troops were still concentrated in multi-legion armies and were not committed to territorial defense, so they were inherently mobile and freely re-deployable. This flexibility resulted in vast disposable military strength, which could be used for further expansion were the front remained “open,” as in Germany before A.D. 9 or Britain under Claudius. Owing to its hegemonic nature, the sphere of imperial control was limited only by the range at which others perceived Roman power as compelling obedience. The reach of Roman power and the costs of its military forces, therefore, did not need to be proportional. Further extensions of the empire, in a hegemonic mode, did not require increases in the military forces maintained. New clients added to the empire could be expected to respond to the same compulsion as did all the clients brought within the sphere of imperial control before them. Hence the economy of force and efficiency enjoyed by the Republican Empire. But this was a system whose goal was to enhance the security of the Roman heartland rather than to provide comprehensive, day-to-day security of the imperial territory and its populations.
The Antoine system, in use in one form or another from the Flavian era after A.D. 69 to the political crisis of the mid-3rd century, reflected the territorialization of the empire and the reorientation of its priorities. Defense was now based on “scientific” frontiers and preclusive (perimeter) defense. Armed forces were now everywhere deployed to secure the tranquility and therefore the prosperity of border lands, and a fortiori, of the interior. The military strength of the empire and its effective power were rigidly proportional, since strength was now largely used directly, not as a tool of political suasion. Clients remained, but they were much less useful than in the past: the task of maintaining territorial security was effectively shifted from weak clients to widely distributed frontier forces, while strong clients could no longer be tolerated, since their strength might now dangerously exceed that of the adjacent imperial forces, which were often stretched thin, at least on paper.
Nevertheless, the empire remained strong, and not the least of its strength was political. A real growing prosperity and a voluntary Romanization largely overcame the last vestiges of nativist disaffection and created a strong base of support for the unitary regime. Facing enemies widely separated from one another at the periphery, the empire could still send overwhelming powerful forces against them, since the tranquility of the provinces – and in places, elaborate border defense infrastructures – allowed peace to be temporarily maintained even with much-depleted frontier forces. The residual offensive capability was primarily useful as a diplomatic instrument; its latent threat served to keep the neighbors of the empire divided, if not necessarily obedient.
Nevertheless, the cultural and economic influence of Rome on the lives of all the neighbors of the Empire itself created a cultural and political bias for common action against it. Men who had nothing in common now acquired elements of a culture shared by all but belonging to none. Beyond the Rhine, the federation of border peoples that eventually turned into formidable multi-tribal agglomerations had begun. Opposed by the relentless force of cultural transformation, Roman diplomacy became less effective in keeping the enemies of the empire divided; and the system of perimeter defense, keyed to low-intensity threats, could not adequately contend with their unity.
The third system arose in response to the intractable combination of diplomacy and military problems whose consequences became manifest in the great crisis of the third century. The Romans devised an elastic defense, in which ad hoc field armies fought against barbarian coalitions deep within imperial territory; and later, under Diocletian, a shallow and structured defense in depth. These new defensive variants were not thought to be desirable. When local threats were defeated or suppressed, every attempt was made to return to the system of preclusive security.
Like the Antoine frontier establishment, the new system provided no short-term disposable surplus of military power either for offensive use or for diplomatic coercion, deterrent or compellent. The difference was that the this system no longer had a surge capability either, since the enemies of the empire were no longer kept on the defense by forward defensive tactics; instead, they were only contained. When the containment forces were reduced to muster ad hoc field forces, enemy penetrations occurred, and the remnant capacity to generate the image of power for the purposes of political suasion were irretrievably lost. The diplomatic relationships with external powers now reflected the local balance of forces, which did not always favor the empire on every sector of the perimeter.
With this, the output and input of the system were finally equated. The level of security provided became directly proportional to the amount of the resources expended on the army and on frontier fortifications. The great economy of force that made the unitary empire such an efficient provider of security was lost. It merely employed certain modest economics of scale over the alternative of independent regional states. And these economies of scale were not enough to compensate for the growing administrative inefficiency and venality. Loyalty to the idea of empire, and fear of the unknown, kept the system in place until organized barbarian states, capable of providing a measure of security in lands that had once been Roman, came to the fore.