In the Republican strategy, which ran to the time of Claudius, armed forces were distributed in pockets around the perimeter of Roman territory. Troops were garrisoned in multi-legion armies which were not intended for territorial defense of the frontier. No Roman troops were deployed in client states but there was constant diplomacy going on between Rome and the clients it sought to influence. The primary aim of the diplomacy was to remind the client of the infinite Roman power and the uselessness of trying to oppose her. This effort of deterrence was part of a “carrot and stick” philosophy where the carrot consisted of subsidies provided to influential persons.
The legions were mobile and freely deployable to any location requiring military intervention, so that major rebellions, like Illyricum in 9 A.D, could call for sending half the army without risking the security of the frontier. Absent rebellions, the deployed legions could also be used to advance the frontier where it was open, as in the case of Great Britain. In the Republican model, the reach of Roman power and the costs of operating the army were not proportional because the psychological threat of the Roman Army greatly exceeded that of the physical army.
The second, or Antonine System, was in use from about 69 A.D. to the middle of the third century. Here the Empire saw its army deployed everywhere to protect and secure the borders and the interior. The effective power and actual power were now in strict proportion because the psychological threat was backed up by large forces that could enforce it. Weak clients were ignored because they provided no value. Strong clients could not be tolerated because their strength posed a threat to the Empire.
Politics was now critical because growing prosperity and the Romanization of the frontier were fostering enculturation. Meanwhile, the threat of facing enemies was low because of their separation from each other. Elaborate border defenses, such as Hadrian’s Wall took the place of large troop deployments and allowed the army to operate with fewer units.
In spite of these positive aspects, there was a growing dissatisfaction among people outside the territories because their adoption of Roman ways was not accompanied by the rights associated with citizenship. As a result, the enculturation created a kinship among the disadvantaged. German tribes now took center stage as their drive to unification became stronger than the influence of Rome. The perimeter defense was not adequate to resist these large unified groups.
The Diocletian system arose in the middle of the third century when the political and military problems Rome was facing posed a significant threat to her stability. The new structure utilized a shallow “defense in depth” strategy to replace the massed force model. As in the Antonine system, there was no surplus of military power for offensive use or deterrent, but unlike its predecessor, the Diocletian system had no “surge” capability.
The concept of defense was one of containment. Because forces had to be put together ad hoc according to circumstances, penetrations were commonplace and the threat of Roman power was everywhere absent. Any diplomacy undertaken with external powers was local and dependent on the availability of forces. The outputs and inputs to the military system had finally come into balance and security was equal to the size of the force put to the task of maintaining it.
At this point the empire enjoyed only a small economy of scale advantage over the alternative of independent regional states but that advantage was counteracted by the inefficiency of administration far from home. The reality of a unified all powerful Empire was now a memory outside of the will of those in power who endeavored to keep the Empire intact.