125 - The Best Defence is a Good Defence

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Hello and welcome to the History of Rome, episode 125, The Best Defense is a Good Defense. Over the centuries, the Roman legions had been transformed and then re-transformed, any time the old way of getting things done stopped getting things done. The Romans loved their traditions, but they never for a second allowed sentimentality to get in the way of always having the best strategic and tactical military operation in the world. For years, Rome was just another city-state, fielding small armies composed of farmer soldiers who fought using the traditional Greek phalanx. When the phalanx proved unworkable in the hills of Samnium, the Romans adapted the legions into the famous three-line maniple system that they then used to conquer most of the Mediterranean. Then Marius came along and revolutionized not just how the Romans fought, but who was doing the fighting, ushering out the age of the temporary conscript army and ushering in the age of the professional standing army. This, of course, wound up having the unintended side effect of also revolutionizing who these new standing armies fought, because for practically the whole of the next century, the enemy the Roman legions fought more than any other was other Roman legions.

Following the triumph of Augustus, the mission of the legions changed again. No longer was their primary objective to conquer foreign lands, but instead it became to protect the lands that had already been conquered. Legionary garrisons were posted on the frontiers to monitor the borders and that is where they basically stayed for the next three centuries. Roman and Hadrian cemented this mission in the late 1st century and early 2nd century AD by literally building walls to demarcate the empire from the barbarian lands beyond. You'll recall from earlier, though, that these walls were not designed to act as insurmountable fortifications, but rather were erected to control the flow of traffic and serve as a stable line from which the legions could mount forward operations and smother potential threats before they actually reached the empire.

The utility of these lines, though, was premised on the notion that the Romans would always have superiority of numbers in the local theaters of operation. As long as the tribes along the Danube or the Rhine or up in Britain were divided from one another, one random chief getting it in his head to cause trouble was no big deal. Send in a legion, smack them around, be home by dinner. But by the late 2nd century, this assumption of numerical superiority became a dangerous fallacy. The tribes were beginning to join forces, something Marcus Aurelius learned full well during the Marcomannic Wars. As the 2nd century gave way to the 3rd century, this evolution of the Germanic tribes from disparate groups into larger confederations only sped up and for most of the next century, as we saw ad nauseum in the last 15 episodes or so, the local legions stopped being a match for the invasion forces that now vastly outnumbered them. The legions and the provinces they protected were overrun, and only through immense effort and more than a little luck was the empire eventually able to turn the invaders back.

For almost the entirety of the 3rd century, this shift in the balance of power forced the Romans to play catch-up, always one, two, or even three steps behind their enemies. A combination of improvised new strategies, think Gallienus' mobile cavalry, and sheer force of will, think Aurelian, allowed Rome to emerge intact into the late 3rd century. But it was clear that one of these days, they were going to have to completely reassess their military posture. With the ascension of Diocletian, that day finally came. Not only did the new emperor possess the intellectual heft to conceive of something more than just another run of ad hoc responses, but when he came to power in the mid-280s AD, the empire had gotten just enough breathing space that it was open to change. In the north, the tribes of the Rhine and the Danube, and especially the Goths, who had been such a destructive force, had been beaten back by Diocletian's predecessors, and in the east, the Sassanids were temporarily weak and divided following the death of Sharpor. The time to act was now, and Diocletian did not miss his opportunity.

So what did he do? Broadly speaking, there are four things I want to talk about today. First, Diocletian's administrative redistricting that created a ring of small, militarized provinces around the edge of the empire. Second, the final separation of military and civilian career paths. Third, the division of the army into two distinct forces, the legions proper and a new static frontier militia. And finally, how these reforms interacted to create a web of varying depth surrounding the empire that would ensnare and then destroy an invading enemy. This is the so-called defense-in-depth theory proposed by Edward Lutwick, and we will talk both about the model he proposed, as well as the critiques other scholars have levied against it.

Now, before we go on, I should note that all the usual caveats about a lack of good sources for all of this stuff still applies, and that as historians and scholars have cobbled together a picture of Diocletian's military, that they have been forced to rely on historians writing decades and even centuries after the fact. It is, in short, difficult to separate out things that were specifically initiated by Diocletian from things that were merely attributed to him. Difficult to separate out things that were centrally planned from things that were initiated on the local level. And finally, difficult to weed out descriptions of his armies that are 100% free of anachronisms. That being said, the composition and posture of the legions did change during the period of the Tetrarchy, and the items I just outlined represent a fairly conservative estimation of the changes Diocletian undertook.

Back in Episode 89, Provincial Matters, we took a 45-minute-long tour of the Empire that touched on each and every province in existence at the height of Rome's fame and fortune. That tour is now officially obsolete. In the mid-290s AD, most likely right as the Tetrarchy was being formed, Diocletian initiated a redistricting of the Empire on a massive scale. At its most basic level, Diocletian waived his imperial wand and doubled the number of provinces overnight. We will deal more with the administrative implications of his redistricting next week, though no, we will not be going on a tour of the now hundred-odd Roman provinces. And for now, I want to focus solely on the military implications of the new boundary lines.

Diocletian undertook the redrawing of the provincial lines for two main military reasons. First and foremost, by fracturing the size of the provinces, Diocletian hoped to cut down on the amount of power a single commander would wield. No more Governor of Britain controlling three legions by himself, or the Governor of Lower Germany personally commanding three or four of his own. Smaller provinces meant a local commander would usually have at his disposal what amounted to a single legion. This hopefully meant less threat of revolt. The second reason the provinces were fractured is that Diocletian wanted to divide up the dangerous and perpetually besieged outer rim of the Empire from the softer and more prosperous inner territories. The former would become essentially military districts, while the latter would be allowed to go back to being simply provinces, or, more accurately, undistracted economic engines for the Empire. In essence, Diocletian wanted the ministers in charge of the outer provinces to focus on defense, and the ministers of the inner provinces to focus on the economy. These two factors had to work in tandem for the Empire to thrive, and the Emperor no longer wanted economic concerns to go unaddressed because the Governor was off fighting the Alamanni, and he didn't want the frontier lines to be neglected because the Governor of Pannonia was stuck dealing with a grain distribution issue.

Related to the division of the Empire into what amounted to military and civilian districts was the final severing of military and civilian career paths. This was not a novel idea by any means, but Diocletian brought it to its logical conclusion. Since the reign of Gallienus, the Senate had been denied military postings, which meant, in practice, that key provincial governorships were now handed over to military men, rather than rich aristocrats, as had been the previous custom. But this was not enough for Diocletian. In his mind, it was just as bad for career soldiers to be running administrative offices as it was for untrained senators to be leading armies. It was an inefficient misallocation of talent, and he aimed to put a stop to it. So, flying in the face of the whole of Roman history, Diocletian decreed that the political governor of a province, and its chief military officer, would no longer be the same man. He did this for the same basic reason he divided up the provinces. First, the checks and balances created by the separation of military and political power would undercut potential rebellion, and second, it would allow each leader to focus on his appointed tasks without being suddenly called off to deal with a foreign invasion or a new tax law. Of course, political and military power could never be divorced from one another at the highest level. But from here on out, on the provincial level, a general, now called a dux, the precursor of the medieval duke, was not in charge of tax collection, and the governor was not in charge of any troops. Together with the creation of the militarized Outer Rim provinces, the split of defense concerns from political or economic concerns meant that the army would no longer be distracted in any way from their all-important task of keeping the empire safe from attack.

The question then is, what did Diocletian's army look like, and how did it differ from the army he inherited? Well, we've already seen that right off the bat he was troubled by the concentration of force around the emperor, and how that left huge swaths of the empire open to attack. That was the main catalyst behind his initial decision to elevate Maximian, and it played a huge part in his further decision to create the Tetrarchy. A sole emperor running around trying to plug every leak in the empire was as dangerous as it was impractical. No matter how fast it could move, it would never be fast enough. So right away, we see the armies return to something resembling their pre-crisis years deployments, only now, each section of the empire was not commanded by a potential usurper, but a fully initiated member of the Imperial Brotherhood. But wait a second, didn't that old system completely break down in the face of immense new pressures? Why, yes it did. But we should not forget that a big reason why it broke down was that usurpers kept leaving the frontiers undefended as they made their bids for power back in Italy. The establishment of the Tetrarchy was supposed to negate that possibility, and at least while Diocletian reigned, that's exactly how it worked.

But of course, usurpers abandoning their post was not the whole story and Diocletian knew it. The other big problem was that the old legionary deployments neither spread the Roman forces out enough to effectively monitor the frontier, nor concentrated enough force in one area for them to overwhelm their new, larger opponents. In other words, the 4800-6000 man Roman legion, which had been in existence for 800 odd years, was now becoming obsolete. Previous emperors had of course realized this already, Gallienus' creation of the mobile cavalry is an obvious result of this understanding, but Diocletian's reformed legionary formations would actually flow from a different tradition. For years, cohorts of soldiers, usually labeled vexillations, would be peeled off from their parent legion and deployed elsewhere to shore up some undermanned area or lend a hand in some difficult situation. Sometimes the vexillations would return home when their specific task was complete, but often they simply never returned to their parent legion at all. In many theaters, these vexillations, usually numbering between 500 and 1000 men, began to be regarded as the de facto unit of deployment. Smaller and faster than a standard legion, they could be bunched together when a concentration of forces was needed, and kept spread apart when the danger had passed and frontier-wide surveillance was needed.

Diocletian knew all about how these vexillation units functioned, and how much flexibility they gave commanders, and when he took power, he aimed to make their status official. That did not mean that the old legions were simply scrapped, and indeed most of the existing legions kept right on functioning as they always had. But it did mean that when Diocletian created new legions, as the circumstances of his times demanded that he do, they were not the traditional size. Diocletian's legions were instead a complement of just 1000 men, which helps explain how he was able to add 30 new legions to the rolls over the course of his reign, which would be a staggering number if you failed to account for their reduced size. These new legions were stationed across the empire at strategic points, far enough apart that they could watch a wide swath of territory, while simultaneously being close enough to one another that if a major invasion broke across the frontier, they could quickly join together and fend off the attack. We'll get into a deeper discussion of that in a minute.

The other real innovation that Diocletian introduced into the army is a little bit trickier to nail down, because it involves the possible insertion of anachronisms into the historical record. The traditional account is that it is at this point that the legions were joined on the frontiers by a less well-trained, less well-paid, quasi-militia called the Limitanii. These troops, drawn from the local population, would be part-time soldiers and part-time farmers, and thus, in addition to helping defend the empire, they would also help keep the heavily garrisoned frontier provinces at least somewhat self-sufficient. Their mission was twofold, first, to police the frontier lines and defend against small-time brigands, and second, to act as a warning bell for the rest of the army if something bigger was on the way. No one expected the Limitanii to stop anything of substance coming across the border, but they were expected to raise the alarm and hold key fortifications until help arrived. The problem with all of this is that though Diocletian usually gets credit for introducing the Limitanii into the system, these militiamen really aren't fully documented until Constantine's reign, leading some to suspect that later historians mislabeled Diocletian's frontier deployments using a vocabulary that had not been invented yet. We have a good idea that small detachments were posted along the frontiers, but the debate over whether these troops represented the same type of Limitanii forces that Constantine used is still a matter of much debate.

What is a matter of less debate is how much all of this increased the overall size of the army. Again, we are working from sources that are further removed from Diocletian's own time as we'd like, but the consensus seems to be that all told, Diocletian took an army roughly 350,000 strong and cranked it up to about 500,000 by the time he retired. He pulled this off by ordering his agents to engage in some pretty heavy-handed recruitment efforts, which is to say they didn't linger too long on persuasion before moving right on into conscription. And of course, if you're wondering what effect this vast increase the size of the military had on the overall economy, as the Romans were essentially transferring 150,000 young men from productive farm labor to non-productive soldiering, well, the answer is that the economy took a pretty substantial hit. A hit we'll get into in more detail in two weeks when we look at Diocletian's economic policies.

The other matter that is still hotly debated today is how all of this fit together. Back in 1976, Edward Lutwick advanced a fairly comprehensive theory of what he called defense in depth. What he argued is that Diocletian essentially gave up trying to defend a static line in the sand, and opted instead to weave a web of defense that an enemy could enter at will, but then only make it so far into, before becoming permanently entangled. The key to this defense in depth strategy was something that we've talked about earlier, but that I haven't mentioned so far today because it wasn't really one of Diocletian's overt reforms, namely, the intense effort the Romans undertook to build strong defensive walls around every city, town, and granary in sight. These fortified positions became key hinge points for the new defensive strategy, and the reason why the whole system was able to function as conceived.

The way it worked was this. Out on the front line, the Limitanei forces would keep an eye on things from a series of small forts connected by a military road. A little bit behind them would be stationed fewer, but larger contingents of cavalry, and a little bit behind them would be one or two main legionary garrisons. A large invasion force would then be able to easily blow past the Limitanei, but that was okay, because they could take refuge in their well-supplied, easily defended forts. Then the invasion force had two options. If the invading army chose to use the Roman roads for easy travel, they would pretty soon hit one of the larger fortified camps, and be forced to choose between besieging it, which would cost them time and resources, or they could bypass it, leaving themselves open to attack from the rear. If on the other hand they abandoned the road completely to avoid either of these scenarios, their pace would then be slowed considerably, usually preventing them from making it very much further before they were met by the Roman response. In any event, the enemy would find no easy access to provisions, since the Romans would have all of those locked up behind the walls of their forts, and quickly either retreat out of frustration, or stick around long enough for every Roman soldier in the region to collapse on them simultaneously from all sides. Thus, would the invaders be repelled quickly and efficiently.

Now is this really the way things worked? The answer is, we have no idea. Some archaeological evidence, particularly in the East, seems to support Lutwik's theory, while the rest seems either inconclusive or downright hostile to the notion of a centrally planned defensive web surrounding the empire. Plus, it has been argued persuasively that whatever else you can say about the Romans, you can never say that they just wanted to sit back and wait for attack. True, during the crisis years they were constantly sprinting from this invasion to that, but except for that period, the Romans always, always wanted to take the fight into enemy territory, and it would have been anathema to their most basic ideologies of warfare to design a system that was premised on a defensive posture. We have ourselves just gone through campaigns on the far side of the Rhine by Diocletian and Maximian, and on the far side of the Tigris by Galerius, both undertaken to preempt possible invasion into the empire. In neither of those cases were the emperors willing to just sit back and catch the enemy in a web.

Debate still rages about what exactly Diocletian's overall strategy was, and indeed, whether or not he even had an overall strategy, or whether this was all just randomly hashed out at the local level. But during his reign, we do seem to see wider and deeper force deployment patterns, an increase in the total number of troops, and a concerted effort to create a militarized ring around the empire, all of which suggest that he did have some kind of long-term defensive strategy in mind for the empire. Unfortunately, as with so much of Diocletian's work, the coming civil wars of the Tetrarchy, civil wars Diocletian had tried so hard to prevent, would wind up overwhelming many of his reforms to the military structure, as Rome once again fell back into the familiar pattern of waging eternal war on itself.

What we can say for sure, though, is that Diocletian helped the empire transition its military from its classical configuration into the force that would be familiar to students of late antiquity, a force that in time set the stage for the armies that would be familiar to students of the Middle Ages. Rome is already practically running its military out of castles, so can King Arthur really be that far behind?

Next week, we will get into one of the other ways that Diocletian set the stage for the next thousand years of history, by discussing his reorganization of the imperial governmental apparatus. I will talk more about his reordering of the provinces and some of the non-military aspects of the shift, as well as introduce a new term that we should all be familiar with today, the diocese, those overarching administrative zones that were taken over by the clergy as the empire in the West was falling apart, and whose borders, in many cases, have remained unchanged down to the present day.

One last thing I'll mention before we go is that, as of today, there are still 7 spots available for the final tour we have scheduled, which runs from May 28 to June 7, 2011. So if you thought you had missed out, or maybe you've been sitting on the fence about coming, by all means, drop by historyofrometour.com, check it out, and sign up. It's going to be really, really fun.