SCRIPT - Defensive Strategies of the Roman Empire (18/12/2017)

Defensive Strategies of the Roman Empire

This video will describe the strategies employed by the Roman Empire to defend its territory, as laid out in American strategist Edward Luttwak’s Book, The Grand Strategy of the Roman Empire. Luttwak is not a full-time historian and archaeological evidence contradicts some of his views, but at a minimum the video illustrates his thinking on several variants of defensive strategy, their pros and cons, and the links that exist between politics and strategy formulation.

 

Generally, an ‘optimal strategy’ is seen as one that gives the most benefit for the least cost, but what counts as ‘benefit’ or ‘cost’ itself depends on circumstance and the perspectives of strategy-makers. Changes in either quickly invalidate old methods, and in that vein Luttwak argues that between 27BC to around 350AD, three strategies were devised to defend Rome. 

Geography of the Roman Empire

Geographically, all three strategies confronted the same problem. As a ‘hollow ring’ of territory around the Mediterranean, the Roman Empire did not benefit from interior lines of communications, as far as land transport was concerned. A legion took 2 months to march from Germany to Italy, and 4 months from Italy to Syria, which meant that Roman legions could not have a rapid enough response time to reinforce ‘crisis points’ on a whim.

There was sea transport, of course, but this was unavailable during the stormy winter and unreliable the rest of the time. Generally speaking, it was only worthwhile if times could be reduced by at least 50%, which eliminated most redeployments not between Europe and the East.

The issue of interior lines would not be significant if the Romans had sufficient force to deal with any regional threats. But the restrictive nature of Roman citizenship and by extension legionary manpower, as well as the cost and risk involved with putting armies under distant commanders, put constraints on the size of the Roman army. So from the 1^st to the 3^rd centuries, Rome’s military power consisted of 28-30 legions totaling about 150,000 legionaries and the same number of auxilia, and it was with this force that Roman leaders sought to secure 6,000 km of border.

 I. The Julio-Claudian System: Partial Security and the Client System

The early Emperors confronted a landscape dominated by the Late Republic’s dramatic expansion, which had added Northern Spain, Gaul, the Danube Valley, Numidia and most of the Roman East to what was now the Empire. In order to properly garrison these regions against revolts and hostile neighbors, the army was compelled to spread itself out; but doing so would eliminate the possibility of further expansion, and perhaps even project Roman weakness and invite foreign invasion. The Romans wanted the best of both worlds – internal security and external expansion – and in order to get it, they used the client-state system that was a holdover from the Republican period.

Sited on the frontiers or, in the case of client-tribes, beyond the frontier, client-states were a key part of early Imperial border security. With their limited military and diplomatic autonomy, clients were to deal with low-intensity raiding and control their unruly neighbors. Against high-intensity invasions, they would absorb the initial blows and then contribute towards the Roman response. In either case, clients were to sacrifice their own wealth and well-being first, far before any Roman wealth, well-being and most importantly, prestige were put on the line. Their reward for all this could be as simple as yet another delay of their annexation.

With clients managing the frontiers, Roman forces were therefore freed from garrison duty, increasing the disposable force Rome had to deter enemies and acquire more clients, further bolstering border security and thus creating a positive feedback loop. Of course, one could lose control of powerful clients, but so long as Rome maintained ‘escalation dominance’ by ensuring that any rebellion would be comprehensively defeated through force of Roman arms, the client system as a whole could contribute positively to Roman security.

The client system’s economy of force also aided continued Imperial expansion: post-conquest, security burdens that would normally tie down garrisons were instead transferred to local clients, freeing up Roman forces for the next campaign. Thus, Roman power extended much further than the size and mobility of its army would initially suggest.

For example, in an Empire possessing only 28 legions, Tiberius in AD 6 was able to use 12 to wage war on the Marcomanni along the Rhine and Danube frontiers. While this move resulted in a strategic vacuum that was filled in by the Illyrian Revolt, a Dacian invasion and finally Varus’ defeat at Teutoberg Forest, the key point here was that the Romans were able to pit almost half their army against a single enemy, because, thanks to clients, they only needed 9 legions to hold down the entirety of the Roman East and North Africa. A similar dynamic could be seen in the Roman conquest of Britain, which saw the commitment of 4 legions without much strategic effect on the Empire’s other frontiers.

The client system had its costs, however, and autonomy naturally meant conceding some fiscal and political rights to chieftains and kings. Clients required surveillance and management to ensure they did their jobs, stayed loyal and didn’t cause extra trouble, and this was not something every Emperor was capable of doing. But even at their best, clients could not provide the security that Roman citizens expected of their rulers, so the system was only politically acceptable in un-Romanized areas. It was one thing for foreign subjects to suffer raids and brigands due to client incompetence; it was another thing entirely for Romans, or Romanized peoples, to be subject to the same indignities.

Finally, one should also note that not every entity could be a client: they had to be somewhat settled and not completely decentralized to be useful, which probably puts the Germanic tribes at the extreme of client-ability.

Thus the early Imperial client system, while efficient, was nevertheless a product of specific political circumstances. Once these circumstances changed in the late 1^stCentury, the strategy behind Roman defence had to change as well.

II. The Antonine System: Comprehensive Security through Border Adjustments

As early as Tiberius, but especially after Vespasian, Rome began breaking down the client system in favor of direct rule. The extra tax was welcome, but Vespasian also realized that clients could pose a danger to Imperial rule, given that his rise to power was aided by the Eastern clients. In any case, few neighbors could now be turned into clients, with Rome now adjoining established states such as Dacia, Parthia, and the condominium of Armenia. In Western Europe, the process of Romanization was well underway, and it was no longer possible to rely on the second-grade security provided by clients for these subjects, especially as doing so could prompt them to support usurpers such as Vitellius in Germania.

Without clients, Rome would be solely responsible for security provision. Tactically, the Empire would leverage its military superiority to preempt enemy threats before they reached the minimally-fortified frontier. Strategically, however, this would mean that the Roman army would spread itself out in a way that would cover all imperial frontiers, sacrificing its disposable force and foregoing any more continuous, large-scale expansion. More legions could be drawn from the now-pacified interior, but Rome would still have to contend with the Empire’s unfavorable geography and its army’s lack of strategic mobility.

Thus with the goal of comprehensive security in mind, Emperors began a campaign of border adjustment, seeking frontiers which would generate the most defensiveness at least cost. Not all of the new borders resulted in fortifications or – in the Syrian or Saharan deserts, garrisoning key oases was enough – but in all cases, the goal was to repel all low- and medium-intensity threats that could conceivably disrupt Pax Romana.

‘Defensiveness’ was defined in two ways. Firstly, borders were set with direct considerations of terrain, redeployment distances, and smoothing out salients in mind. One example of this could be seen in Southern Germany: prior to 70AD, the frontier followed the natural line of the Rhine and Danube, which resulted in a 170-mile wedge centered on the Neckar Valley – clearly an ideal route for an invader heading towards the Alpine passes to take. Over a hundred-year period, Emperors reduced the salient in phases, anchoring the northern edge at the Taunus Mountains and delineating a roughly L-shaped border from Mainz to Nuremberg. Not only did this new border shorten Germania-Moesia redeployment times by 10 days, forts on the Taunus could also give advance warning regarding potential westward invasions of the Germanic tribes. Septimius Severus’ establishment of a Roman salient at the head of the Euphrates and Tigris could also be seen in this light, since it added depth to Roman Syria, provided a safe route into Parthian Mesopotamia, as well as outflanked any Parthian westward invasions.

Secondly and less obviously, ‘defensiveness’ also took into account the effects that the new borders had on neighboring entities. A positive example of this would be Dacia, obtained as part of Trajan’s conquering spree. Despite it being a huge salient, Rome retained Dacia as eliminating the Dacians removed a serious competitor for influence over the Central European tribes. Doing so also separated the Danubian Sarmatians from each other, keeping them weak enough to come under Roman control and thus indirectly shielding the Balkans. On the other hand, a negative example could be abandoning Scotland and later the Antonine Wall in favor of Hadrian’s Wall, due to the difficulty of controlling the unruly tribes there.

No matter the defensiveness of the new borders, the spreading out of the Roman Empire, as demanded by the strategy of comprehensive security, carried with it distinct costs. Rome no longer had the disposable force to force compliance beyond its borders, which meant that Roman security now depended on the strength of the legions on any given sector. If the legion responsible was redeployed or defeated, the resultant security vacuum could result in a disastrous self-reinforcing loop, wherein new troops would have to be redeployed in order to counter invasion, thus creating more security vacuums, and so on. Trajan’s Mesopotamian campaign, for example, ended in near-disaster as he stripped the Roman East of troops, which allowed the Jews, which in the earlier system had been pacified through Herod’s client kingdom, to revolt for the second time in 50 years.

The existence of this potential feedback loop meant that the Empire’s pre-deployment of its forces was of paramount importance. As the 3^rd Century dawned, Rome’s forces were still arranged in a way that presupposed unorganized tribes on its European frontiers and a weak Parthia to its East. The former assumption was already coming apart with Marcus Aurelius’ Marcomannic Wars, and after Parthia was replaced by the Sassanid Empire in the 220s, the system was tested to destruction.

III. The Late Imperial System: Imperial Security and Defence-in-Depth (or was it?)

Beginning in 230, Sassanid Persia began the first of a long series of wars with Rome. Facing, for the first time in a while, a centralized enemy threatening the entirety of the Roman East, legion after legion was drawn from Europe to meet high-intensity Sassanid assaults. Security vacuums began to appear in Rome’s European frontiers, and were quickly filled by Germanic and Gothic invasions.

Even without foreign invasion, the fragility of the Imperial structure was fully laid bare during the Crisis of the Third Century. With civil war, assassination, and usurpation the order of the day, the Emperor’s priorities became detached from the Empire’s priorities. It was no longer prudent for an Emperor to assign large forces to guard distant frontiers; he needed them close to himself.

The result was the expansion of imperial guards into central field armies, a strategically wasteful invention given the army’s low mobility. Thus, despite a ballooning of the late Roman army to 500-700 thousand soldiers, invasions continued to occur on a near-regular basis as weak frontier garrisons were no longer a match for the increasingly developed Germanic and Gothic confederations.

Luttwak argues that this combination of regular invasion and politically-necessary central field armies found its logical strategy in a ‘defence-in-depth’ system. Rather than previous systems, which sought to prevent large-scale invasion into Roman territory, defence-in-depth accepts this as a fact and aims to contain the enemy, exhaust it and eventually defeat it with reinforcing units. In Roman terms, Luttwak outlines a system of local forts, held by semi-professional but territorialized forces, and a goal of delaying or denying the enemy for as long as possible before reinforcements arrive on the scene.

The success of a ‘defence-in-depth’ ultimately still relies on the success of the field army. If it is destroyed – like at Abrittus in 251 or at Adrianople at 378 – then the various obstacles would only delay the inevitable, and for the Romans there would have been little else to do but to pay the enemy off.

Luttwak’s ‘defence in depth’ system has never been supported by archaeological evidence, which suggests that the Romans never really abandoned the goal of comprehensive security, and instead were slowly overwhelmed by simultaneous pressure from Persia and Germania, and later the Hunnic and Gothic invasions. Certainly, even Luttwak agrees that in periods of stability such as under Diocletian or Constantine, the Romans re-concentrated their forces on their frontiers, consistent with the idea of comprehensive defence.

Both systems of ‘defence-in-depth’ and comprehensive depth would have been in line with the strategic retreat conducted by late Rome. Rome no longer had real control over neighboring peoples, and given the desperate situation during the Crisis, forward positions had to be abandoned in order to reduce imperial overstretch. In such a way, Rome retreated from forward positions such as Numidia and Southern Germany and abandoned the exposed salient of Dacia.

Finally, ‘defence-in-depth’ views the strategic costs that the Empire accepted during the period as part of a broader strategy, as opposed to under comprehensive security where it largely comes about due to policy failure. One could argue that, from the perspective of the average Roman citizen, the question was moot: should the repeated ransacking, evacuations and general unhappiness not be enough to reduce faith in the Empire as an effective provider of security, then the high taxes needed for new forts, armies and tributes would clearly do it, and it wouldn’t matter much if this came about due to a hardhearted Imperial decision to trade land for time or because the Empire was unable to hold the line.

Still, such costs were bearable if the alternative was anarchy. But once the Germans and Goths became providers of even rudimentary governance, then the incentive to maintain the onerous Imperial structure quickly fell away. This may explain the survival of the Eastern Empire, which had a about 60% of its force in territorial defence, compared with the Western Empire, which devoted 40% to defending its citizens.

Conclusion

It is easy, especially for those accustomed to economic rationalism or cost-benefit analysis, to see the client-system as an ‘optimal’ strategy. But this overlooks the political circumstances that made such a system viable in the first place. Similarly, one could criticize the development of central field armies in the third system was strategically wasteful, but it is unreasonable for Emperors not to consider their personal security when making any decision during that turbulent time. Strategy divorced from political realities are ineffective at best and disingenuous at worst.

Turning back to Rome, one could argue that the client system and the comprehensive system addressed the security needs of the Empire and, to some extent, the subjects who collectively sustained the structure of the Empire. Conversely, the sacrifice of the broader population’s safety in favor of the Emperor’s safety was a tradeoff that could perhaps be justified in the medium-term, but grew increasingly self-serving and divorced from political demands as security issues remained unaddressed. Ultimately, once rival states found cheaper and more effective security strategies, the rationale for keeping the Western Empire afloat faded away and Rome met its doom.

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