Byzantium’s Eastern Frontier: The Most Sophisticated Defensive System of the Middle Ages

Below is the transcript for the History Network Podcast episode “Byzantium’s Eastern Frontier”. You can listen to it here, or on iTunes or Spotify. The episode was based on this Twitter thread.

The Byzantines, the subjects of the Eastern Roman Empire, were great survivors. They outlasted their cousins in the west by a thousand years, withstanding the great waves of barbarian invasions and even managing to flourish amidst the chaos. Less than a century after the last western emperor was deposed in 476, the Eastern Romans under Justinian reconquered Italy and North Africa, and seemed on their way to restoring the entire Mediterranean to Roman rule.

Yet much of this early good fortune was illusory. More calamities were in store: plague, new waves of invaders, and economic collapse wiped away much of their gains and undermined the foundations of their rule. Amidst this fresh crop of disasters, one challenge above all else threatened to bring down the empire: the rise of Islam.

The great Arab conquests began in earnest in 633, just a year after the death of Mohammad. Muslim armies exploded out of the Arabian Peninsula, and in a few short years overran most of the Roman east: Syria, Palestine, Egypt, and Mesopotamia. Soon they were pressing into Asia Minor, sacking cities, pillaging the countryside, and occupying territory.

While this was unfolding in the east, there was ongoing trouble in the west. Nomads from the steppes crossed the Danube and raided the Balkans, establishing their own states on Byzantine territories. Slavic invaders began settling in parts of Greece and the western Balkans. The empire was collapsing on all sides.

Byzantine fortunes reached their nadir during the great siege of Constantinople of 717. That year, an Arab fleet appeared before the city and discharged an enormous army which began a year-long siege of the Byzantine capital. The city was only saved by its magnificent fortifications, which allowed it to hold out until the attackers ran out of supplies.

After these dark days of the 7th and 8th centuries, Byzantine fortunes began to turn. The empire underwent a cultural and military revival over the next 300 years that brought it to a new zenith. By the 1040s, the borders of the empire encompassed modern Turkey, Greece, and all of the Balkans, plus parts of southern Italy.

How did they manage this? The challenge was much more daunting than their earlier recovery under Justinian, as they were starting from a much weaker position. They had far less territory at the outset, and their enemies were comparatively much stronger—in the Balkans, the Bulgarians had set up a powerful kingdom on their doorstep, and in the east, the Islamic Caliphate was a large, powerful empire that also had mastery of the seas.

By the same measure, the Byzantines still held several advantages. The heartland of their empire—western Asia Minor and Thrace—were fertile and populous, giving them enough wealth and manpower to keep up the fight.

They also had a magnificently situated capital. Constantinople was both highly defensible and well positioned for commerce, profiting from the trade routes that crossed the Mediterranean and came overland from Asia.

Finally, the empire’s population by this point was very homogenous, consisting almost entirely of Greek-speaking Orthodox Christians. This gave it the cohesion and morale to hold together in even the direst of circumstances.

Following the siege of Constantinople, the frontier between Byzantium and the Caliphate stabilized in a zone along the Taurus and Anti-Taurus Mountains. These formidable ranges in southeastern Turkey separate the highlands of Anatolia from the plains of Syria and Mesopotamia. Although much of the Arabs’ offensive spirit had been dashed against the walls of Constantinople, they continued to launch large-scale incursions into Anatolia. These forays were economically devastating and occasionally managed to seize cities and fortresses. This was dangerous—if the Arabs managed to gain a permanent foothold beyond the mountains, they might well turn their full attention back to conquest.

The Byzantines’ challenge was therefore to hold off the pressure in Asia Minor long enough to restore their economy and recover their European territories; this in turn would eventually allow them to deal with the eastern menace more permanently. This strategy required patience and foresight—it was a project of generations, not years. The fruit of this long project was the most sophisticated defensive system of the Middle Ages. The Byzantines’ eastern frontier was a complex machine, making use of fortifications, soldiers, civilians, and terrain to grind down a stronger adversary. Although it was centuries in the making, it proved resoundingly successful in the end.

Before we look at how the Byzantine frontier functioned, it is worth a brief diversion into how frontier defenses have typically been arrayed throughout history. Generally speaking, there are three basic forms of defense: linear, defense-in-depth, and mobile. Each has its own strengths and weaknesses and is best suited to particular circumstances.

The linear defense is exactly what it sounds like, a long line of forts, walls, and natural barriers—think World War I trenches, the Great Wall of China, and the Roman limes along the Rhine and Danube. Defensive forces are mostly static, stationed in fixed positions, but are often backed up by a mobile reserve that than can rush to a point of crisis.

Linear defenses have to cover the entire length of a frontier in order to work, which makes them very expensive to maintain. They are usually only employed by great superpowers such as Rome or China, or along narrow, oft-contested fronts, such as those parts of Flanders that separated French and Habsburg territories in the early modern period.

The second scheme, defense-in-depth, sacrifices the strong boundaries of a linear defense in favor of a multitude of strongpoints that stretch deep into friendly territory. It works by forcing the enemy army to march through a thicket of fortresses and other obstacles that slows him down and saps his strength until his momentum is exhausted. Every fort along the way has to be reduced, which takes time, covered, which takes manpower, or bypassed, which exposes the army to attacks from its rear. Crops would be gathered behind strong walls to force the enemy to disperse over a wide area in order to forage. Defense-in-depth was the strategy of several medieval kingdoms in Western Europe, whose landscapes were dotted with castles.

A mobile defense, in contrast to the linear and defense-in-depth, is far more flexible. The bulk of forces are kept in the field armies which march out and meet the invaders in open country. Detachments might be prepositioned in fortresses along likely invasion routes, but this was only to delay the enemy enough for the main army to assemble. This was what the Byzantines used on their western frontier. Whenever the Bulgarians or some other people invaded, the army would assemble and march out to fight them off.

A variant of the mobile defense is the elastic defense. The defending army falls back under pressure, only stopping to harass and delay the enemy. As the invasion progresses, enemy supply lines get longer while the defender’s get shorter, while also operating on friendly ground. Eventually, the advantage shifts so far to the defender that he launches a devastating counterattack, snapping back like an elastic band—exactly what the Russians did against Napoleon and Hitler.

This type of defense only works when the defender has plenty of ground to give. For countries not quite as vast as Russia, the defender can augment his mobile forces with a defense-in-depth, dissipating the enemy’s strength in preparation for the counterattack.

This last point illustrates the fact that none of these defensive schemes are pure. All three make use of both fixed positions and mobile forces, and usually provide some protection on the frontier and in the interior; what distinguishes them is a matter of emphasis. States favored one over the others based on the threat they faced and their own capabilities. Which brings us back to Byzantium’s eastern frontier. What made this unique was that it employed all three types of defense in a phased systematic manner. It was this complex and well-coordinated scheme that broke the momentum of three centuries of invasions, allowing the Byzantines to resume the offensive.

Any defense, no matter which type, has to account for geography. The centerpiece of the Byzantines’ defense was the Anatolian plateau, rugged highlands that are too desolate to supply large troop formations for any length of time. This created problems for both invaders and defenders, but gave a comparative advantage to the Arabs, whose mobile style of desert warfare was well suited to the environment. They attacked in quick-moving columns and lived off the land, moving on to the next region before local supplies were exhausted. Since the Byzantines could not maintain large numbers of troops in a single area, they could not easily bring the enemy to battle: by the time they managed to concentrate their own field armies, the enemy had already moved on. Moreover, for much of this period they lacked the manpower to assemble sufficient forces in the first place.

Which raised another problem. Although the Arab columns were not especially large by the standards of conventional warfare—usually just a few thousand men—they were positively massive for a raiding force. Since these excursions were opportunistic by their very nature, the great worry was that they would catch a town unprepared and decide to hold onto it, incrementally expanding their territory year by year. Worse, they might surprise the Byzantines by launching a much larger expedition than usual to deliberately target a major city near the border. It was therefore critical not just to head these incursions off, but to stop them altogether if possible. Through long, bitter experience and much trial-and-error, they gradually worked out a winning strategy.

Arab raiding expeditions usually assembled in northern Syria around late August or September. This was just before the harvest in Anatolia, when the summer heat was starting to break—ideal marauding season. Fighting men gathered from all over the Islamic world, from as far as Egypt and Persia, drawn by the promise of plunder. As they assembled, the Byzantines sent scouts and spies to figure out which route the invaders would take. As soon as they found out, frontier commanders rushed infantry to block the high mountain passes along the way—a natural linear defense.

Sometimes this stopped the invasion dead in its tracks; more often, the highly mobile column just found another route. Even so, this gave the Byzantine frontier commander critical intelligence and time to assemble his forces in a suitable location. If the enemy force was not especially large, he would seek out battle immediately, a straightforward mobile defense. If it was more than he could handle, he would put the neighboring regions on alert and start gathering troops from all over. In the event of a serious invasion, he could use a system of warning beacons stretching back to Constantinople itself to summon the imperial army.

Warnings were passed to the local inhabitants as well. Frontier troops were raised from the local population, and this fostered close civil-military cooperation. Centuries of raids had forced Anatolian villagers to harden their settlements, relocating to higher ground, building walls, and developing alert systems to bring crops and livestock to safety when raiders were nearby. This made it harder for the Arabs to loot and forage, often forcing them to waste time and energy on sieges of fortresses and fortified settlements. Even when they were successful, this narrowed their advance, slowed them down, and wore them down—an organic defense-in-depth, in other words.

At a minimum, this mitigated the damage from the raids; when especially effective, it could turn them back altogether. Armies can only go so far without provisions, and men hoping for easy loot could become quickly demoralized. Although this might not sound quite like victory, it had a cumulative effect over time. Raiding parties attracted fighters more by the promise of plunder than loyalty to a particular commander, so the failure of one year’s expedition would blunt the enthusiasm for the next.

While towns and fortresses were preparing their defenses, the Byzantine field armies were mobilizing. They did not do so according to standard practice, whereby the entire army mustered in one location before marching out to meet the enemy. Especially in the early centuries, the Arabs usually outnumbered the Byzantines. The latter learned from bitter experience not to rush headlong into battle, where a single defeat could knock out the bulk of their defensive troops and expose all of Anatolia to the enemy’s depredations.

At first, these field forces would strictly shadow and observe the enemy. Like the Arabs, they were mostly cavalry and could move quickly. As additional troops streamed into the area and their strength grew, they would start to skirmish with the enemy. The Byzantines would only fight when they had a clear advantage, so they launched night attacks and ambushed isolated raiding detachments, using the terrain to their benefit wherever possible. This wore down the invaders and constrained them by their mere presence, preventing raiding detachments from ranging freely across the countryside.

Up to this point, the shadowing force was no more than an auxiliary to the defense in depth. If the Arabs became discouraged at that point and turned back home, the Byzantines could chalk it up as a victory. For much of this period, that was the best they could hope for, as they simply didn’t have the manpower to do more.

But occasionally, the Byzantines were in a position to get more aggressive. The cumulative effect of attrition, fatigue from the campaign, and the encumbrance of spoils made their enemy weaker, less enthusiastic, and slower. As the Arabs were getting worn out, the Byzantines were getting stronger, as fresh reinforcements streamed into their own ranks. At a certain point, they could transition from cautious skirmishing to seeking out full-scale battle—a classic elastic defense.

The Byzantines had every advantage by this point. They were fresh, knew the terrain, and most importantly, could choose the site of battle. If they had the opportunity presented itself, they would rush their infantry ahead of the withdrawing enemy to block the mountain passes along his line of retreat. Following behind with their cavalry, they would then smash the enemy between hammer and anvil. A successful battle would reduce the enemy’s fighting strength in the near term, and if it were decisive enough, it could deter future raids for many years to come.

So to summarize: the Byzantines used a linear defense to block the mountain passes if they could, then use a mobile defense to fight off the raiders. Failing that, they used a defense-in-depth to mitigate the damage and wear the enemy down. Finally, when able, they used an elastic defense to deal the enemy a crushing blow. This complex defensive system required coordination among multiple levels, from the emperor’s armies down to fortress garrisons and the local peasantry. This was by no means a straightforward evolution, but took much experience—and many mistakes—to get right.

The Byzantines’ chief antagonist during most of this period was the Abbasid dynasty, the rulers of the great caliphate centered on Baghdad. During times of intense conflict, the Abbasids usually had the upper hand; when they had weak rulers or were preoccupied with civil strife, the Byzantines could recover and make good their losses. Over the long run, however, the Byzantine military machine was steadily improving; their victories were becoming greater and their defeats less severe.

Abbasid power began to crumble in the later 9th century, eventually to be replaced by regional powers. In Syria, they were succeeded by the Hamdanid dynasty of Aleppo, a leaner and more aggressive state that resumed the offensive in Anatolia. By this time, however, the Byzantine frontier system had come into full maturity. The empire itself was much stronger, with tax revenues flowing into the treasury, greatly expanded borders, and a much larger army. The Byzantines were able to sustain offensives of their own, sending armies into Syria and taking back cities in the borderlands that had long been under Muslim control. They were confident enough in their defenses that, whenever news arrived of an Arab raiding party, they would send their own marauders to plunder the enemy’s undefended countryside.

A series of soldier-emperors in the later 10th century brought the war with the Hamdanids to a successful conclusion. They recovered Antioch, the great metropolis of northwestern Syria, subjected Aleppo to tributary status, and brought much of the coast under Byzantine control for a time. Henceforth Syria would no longer pose a threat to the heartland of the empire.

Yet the Byzantines also knew that fortune is fickle. Success had made their defensive methods obsolete, but it might make itself useful in the future. One emperor commissioned a manual to record these techniques in a manual entitled On Skirmishing. It is our major source for the nuts and bolts of how the Byzantine army operated on the frontier, and it fills out many gaps in the narrative histories of the period.

Unfortunately for Byzantium, these methods did not do them much good in the future. The next big threat in the east would use a different style and come from a different direction: the Turks coming through the Armenian mountains. In 1071, barely a century after the Arabs of Syria were subdued, the Seljuk confederation won a resounding victory at Manzikert. This battle, which took place in the far eastern part of modern Turkey, destroyed much of the Byzantine army and opened up all of Anatolia to Turkish conquest.

Although the empire would recover from that disaster, it never again controlled the highlands that formed the centerpiece of the frontier defenses. For the rest of Byzantium’s history, the border lay along the mountains of western Anatolian rather than the Taurus farther east. Turks of various stripes dominated the plateau itself, giving them easy access to the rich provinces of the coast. This proved a constant drain on the empire, even as new, more dangerous threats arose in the west—Normans, Pechenegs, Venetians, and a revived Bulgarian state.

Despite all this, the empire survived another four hundred years. It was too strong, too rich, and too resilient to be brought down by a single calamity, and the ensuing flurry of disasters only gradually took their toll. The empire took so long to descend from the heights of its 10th and 11th century golden age precisely because that golden age was so hard-won. Every small success was built on the one that preceded it, and the effort involved the whole of society. Byzantium’s zenith was less spectacular than some other empires, but it was far more robust than those fleeting blooms. In this sense, at least, the Byzantines’ incredible frontier system defined the essence of the empire itself.