For Rome, the Sassanids were a threat on a scale the Parthians never were, not only because of their greater military capacity, but also because of their ambition. The Parthians had expended much of their energy defending their positions in Armenia and Mesopotamia. When they did go on the offensive, it was typically to make gains on the margins of the Roman Empire. On a few occasions, when they thought Rome’s emperor was weak or distracted, they would risk launching their armies on deep raids of Roman territory. When they did, cities and fortresses might fall, and the lands the Parthians passed through would be devastated, with great loss of life. But even if the Roman military was slow to react, the raiders never stayed long, for there were always limits to how long what amounted to a feudal army could be kept in the field. Eventually the Parthians would leave, dragging their ill-gotten gains back to their homeland. There was never any serious Parthian program of conquest aiming to replace the Romans in the eastern half of their Empire.
This was not the case with the Sassanids, who, according to Dio and Herodian, claimed a right to rule over every region conquered by Cyrus the Great and his Achaemenid successors in the centuries before Alexander the Great dismantled the first Persian Empire.1 According to Herodian,
The entire continent opposite Europe, separated from it by the Aegean Sea and the Propontic Gulf [Sea of Marmara], and the region called Asia he wished to recover for the Persian empire. Believing these regions to be his by inheritance, he declared that all the countries in that area, including Ionia and Caria, had been ruled by Persian governors, beginning with Cyrus, who first made the Median empire Persian, and ending with Darius, the last of the Persian monarchs, whose kingdom was seized by Alexander the Great. He asserted that it was therefore proper for him to recover for the Persians the kingdom which they had formerly possessed.2
After the death of Artabanus the Parthian nobility was quickly brought to heel, and many of the great families were brought into the imperial court, while Ardashir appointed members of his own family to all of the kingships within the Empire. Unlike Parthian rulers, these kings were never allowed to become powerful warlords capable of straying from central control. With the throne secure, Ardashir considered his position strong enough to engage with the powers on his borders. He appears to have moved first against the Kushan, who controlled a vast empire straddling both sides of the Hindu Kush. The Sassanids were unable to collapse the Kushan state, but it appears that they conquered Bactria (Afghanistan) and may have exterminated Kushan power north of the Kush. Still, throughout the third century and well into the fourth, the Sassanids could never ignore Kushan. Kushan power was never as great or as threatening as Rome’s, but it could be ignored only at some peril. Moreover, though we know almost nothing of other trouble along the Sassanid frontiers, the level of violence must have been great, since most of the tribes migrating toward Rome would first have encountered and tested the strength of the Sassanids.3
In 232 Ardashir felt ready to turn west and face Alexander Severus’s Roman Empire. Most of what followed has already been related in this book’s main narrative. But one big strategic change must be noted here. For over two hundred years the Parthians had been on the strategic defensive, with Roman armies routinely marching deep into the Empire to sack Parthian cities, including their capital at Ctesiphon. From the rise of the Sassanids until Justinian rose to power in Constantinople, the Romans were almost always on the strategic defensive. There would be periodic Roman offensives, but these often met with disaster, and even when there was some measure of Roman success, it was offset by the dissipation of resources that could have been better employed elsewhere. As we shall see, Rome had the strategic answer to the Sassanid threat: fortresses along the main avenues of attack, backed by fortified cities in the interior and Roman legions that could concentrate to defend key points. But having the solution is one thing; adhering to a winning strategy while under pressure to do something more proactive is quite another.
Rome appears never to have understood that the Sassanids presented a fundamentally different level of threat than the Parthians, or that the old methods of dealing with the Persians—invade, sack, pillage—were no longer going to hold the line. In the Sassanids Rome faced an enemy capable of employing more powerful forces, for longer periods, to seize the strategic initiative, which, from at least the reign of Trajan, had lain with the Romans. What saved Roman power in the long run was that the Sassanids, despite claiming to want to reestablish the Achaemenid Empire, never truly had any such plans. How else does one explain the speed with which they abandoned the great city of Antioch after capturing it twice in the third century? Either the Parthians were more interested in booty and carrying off slaves or they realized they could never hold territory so distant from the core of their power and within the second heartland of Roman power. To understand why, one has only to note that the Sassanid army had to carry its supplies across hundreds of miles of desert, while the Romans could use sea transport to mass and supply their forces. Moreover, the Sassanid army was still not a professional force comparable to the Roman army. The Sassanids had centralized their power far more than the previous Parthian rulers, but their army was still more feudal than the long-serving legions. Consequently, there were still limits to how long their forces could stay in the field, since the army was prone to breaking up once it had secured all the loot available for easy taking.
But, in 235 Ardashir captured the cities of Nisibis and Carrhae, and later, in 241, he captured and destroyed the city of Hatra. These combined captures gave the Sassanids control of the terminus of each major approach route to the heart of the Roman Empire and overthrew the defensive frontier established by Septimius Severus. The speed at which these cities fell also demonstrated that the Sassanids had regained the old Persian skills in siege warfare, something the Parthians rarely did very well.
Although the specific impacts of climate during the formation and decline of the Roman Empire are hotly debated by historians and specialists in the field, few doubt that climate had a substantial impact on the Empire, and hence on its developing strategies.4 Several strategically relevant points must be made about climate change during the Roman period. The first is the sheer size of the Roman Empire, which, stretched over three continents, meant that the impact of climate change was never even. Consequently, even if one knows the broad changes in climatic factors, such as temperature and rainfall, these aspects of the problem are not going to tell you much about their impact in North Africa as opposed to Britain. For instance, a slight temperature rise might increase the amount of arid territory in North Africa while increasing rainfall in northern Europe. Moreover, given the Empire’s size, there were always multiple micro-climates that varied considerably from one another and could change in unpredictable ways.
It is also worth noting that the Holocene Period, in which we now live, began at the end of the last Ice Age, approximately 11,500 years ago. Since the second half of that period, which coincides with the birth of civilization, the global temperature has been remarkably stable, never moving more than a few degrees in any direction. Still, within this range there has been tremendous volatility, with temperatures even a few degrees lower creating a mini–Ice Age capable of causing profound societal turmoil.5 On the other hand, an increase of a few degrees has historically been equated with the Roman Climatic Optimum and the Medieval Warm Period, when societies around the globe flourished.6 So, although every specific impact of climate change during the rise and fall of Rome cannot be identified, the overall picture is evident. During what is known as the Roman Climatic Optimum, stretching from 100 bce to about 200 ce, temperatures were two or three degrees warmer than at present. During these three centuries Rome thrived as the economy and trade boomed and even the lot of the Empire’s poor improved markedly. But after 250 ce came a few hundred years of cooling that correlate with Rome’s collapse in the west and the advent of the Dark Ages.7 Within both of these climatic periods, there was still variability: there were years, even decades, of cooling during the Roman Climatic Optimum, as well as the opposite, decades of slight warming interspersed with the centuries-long cooling trend.
Before deciding how Roman strategy could adjust to a post-250 climate crisis, it is crucial to comprehend, that unlike today, when climate variations are continuously tracked by satellite and an array of sensitive terrestrial instrumentation, the changing climate was nearly invisible to the Romans. As the changes took place over multiple lifetimes that included sporadic multi-year reversals, it would have been impossible to identify either a cooling or a warming trend and then attribute it to a natural phenomenon that was likely to stretch out for centuries. What the Romans could see was the direct impact of the weather, but they would have viewed as the work of capricious gods. Droughts and floods were something to be endured, not planned for.
Moreover, although the Pax Romana allowed increased food production, much of this was absorbed by a growing population that left many regions with only a narrow margin of agricultural surplus every year. Thus, even small perturbations in the climate could push some regions to the edge. And even if they could adapt sufficiently to feed themselves, the lack of any surplus denied Rome the taxable resources that underpinned Roman power. Over time societies would adapt to the new conditions, no matter the severity. But they would do so at a lower standard of living, for Roman technology was not advanced enough to help societies adapt in ways that would make any measurable difference in sustaining the agricultural base Rome had enjoyed during the interval of the Climatic Optimum. As agricultural production dwindled, so did Rome’s capacity to absorb shocks and maintain power on the margins of the Empire.
All the available data agree that the rise of Rome coincided with a uniquely stable environment. Even with the generally low level of solar activity until 600 CE—activity that correlates closely to natural climate change—the period from 100 bce to 200 ce stands out as an exceptional stable few centuries. Moreover, it appears that even global tectonics were conspiring with the sun to present Rome with a unique environment for increasing its power, as this period was also remarkable for a lack of volcanic activity. Literary evidence indicates that even the volatile fault that stretches across Anatolia, the Aegean, and deep into the Mediterranean was unusually silent for much of this time.
But starting in 200 ce and accelerating in the middle of the third century, climate becomes increasingly unstable, as compared to the prior three centuries of the Climatic Optimum. Overall, proxy data demonstrates a shift toward a broadly cooler, drier climate, particularly in northern Europe, throughout the tumultuous third century. Even volcanic and earthquake activity picked up through the period, although both eased in the fourth century. But the major volcanic eruptions that are clustered in the middle of the third century were likely triggers for rapid, multi-year climate changes that would have reinforced any ongoing solar forcing. Such rapid changes, even if they persisted for only short periods, would have wreaked havoc with food production across much of the Roman Empire, coinciding precisely with the most difficult crises the Empire had ever faced up till then.8 While evidence for the precise causes of the great migrations of peoples from the vast Asian steppes is lacking, it is almost inconceivable that climate change’s impact on pasture lands was not one of the primary drivers.9
Clearly much work remains to be done to match climatic changes with actual events on the ground. But just as clearly, a changing climate set in motion a series of shocks and events that Rome found increasingly difficult to overcome. Moreover, there was no way for Rome to foresee and plan for the consequences of a changing climate. How could an emperor bring together his consilium to discuss ways to prepare for a predicted lack of rainfall deep within the Eurasian land mass that would impel the Huns to head west, when they had no idea that such a chain of events was even possible? Thus, for much of the second half of the Empire’s existence, strategy became a series of reactions and adaptions to macro-events fundamentally catalyzed by climate change.
For the Romans, disease was a part of life. With no idea what germs were, no sure way to preserve food for extended periods, and clean water always at a premium, repeated exposure to disease kept average life spans low. This in turn stunted the growth and development of much of the population. But this had always been part of the human condition and was therefore deemed unremarkable. Disease was what it had always been, a persistent backdrop to all other activities.10 What was remarkable was novel pestilences and plagues that struck with virtually no warning and spread rapidly through a population. In some cases the pestilence could be localized and deaths kept in the thousands. But in others, thankfully rare, a plague would break out and kill millions through huge swaths of territory.11
What was required for a devastating plague was a bridging mechanism that could bring a deadly pathogen from one germ pool into another whose population had never been exposed to that particular germ or virus before. In the modern era, this is remarkably simple: the world is interconnected as never before, and a new pathogen can cross continents in hours. In the ancient world this speed of transmission was rare, since the great population pools within which pathogens developed were widely separated. Any carrier of a plague had a good chance of dying long before he could make it to another population center. But if a new pathogen did find its way into the Empire, it could move with astonishing speed, because the integration of the Empire’s economy provided the pathways upon which any germ could hitch a ride. And if moving by sea, it could travel almost a hundred miles a day.12
By far the worst epidemic to strike Rome was what is commonly called the Antonine Plague, which struck in 165 ce. It originated somewhere deep in Eurasia, likely China, and moved into the Parthian Empire, where Romans fighting in Mesopotamia caught it and brought it back to the eastern cities. When these soldiers returned to their bases along the Danube, they carried the plague with them into the heart of the Western Empire. The epidemic, most likely smallpox or some ancestral antecedent of it, raged for fifteen years and lingered in some regions for yet another decade. The human cost was horrific, and estimates of the death rate in the Empire range from a low of 10 percent to a high of a quarter of the population. The sources are all in agreement that the legions were particularly hard hit by the plague: “in Rome and throughout Italy and the provinces most people, and almost all soldiers in the army were afflicted by weakness.”13 This alone is sufficient to account for the astounding recruitment measures Marcus Aurelius is recorded to have undertaken to fill the legions facing the Marcomanni along the Danube frontier.
Galen, the most famous doctor in the Roman world, apparently left Rome to escape the plague, but he was recalled by Marcus in 168 to tend to him and the royal household in Aquileia. He recorded his experience:
On my arrival in Aquileia the plague attacked more destructively than ever before, so the emperors fled immediately to Rome with a small force of men. For the rest of us, survival became very difficult for a long time. Most, indeed, died, the effects of the plague being exacerbated by the fact that all this was occurring in the middle of winter.14
It is noteworthy that this level of virulence persisted four years after the plague had first established itself. Just how deadly the plague was is also attested to in Jerome’s Chronicle of 168: “There was such a great plague throughout the whole world that the Roman army was reduced almost to extinction.”15 Our sources can always be counted on to tell us how events impacted the army, since the army was one of the senatorial class’s primary concerns. But the economic impact, not usually an area of interest for ancient writers, also faced catastrophe. We know, for instance, that there was a huge collapse in tree felling throughout northern Europe during and immediately after the plague. This corresponded to a significant curtailment in construction projects and the use of charcoal. The impact on mining may have been even more serious. According to an interesting study of Greenlandic ice cores, there was a dramatic fall in European lead pollution at the time of the plague. The authors of this study concluded that “the Antonine plague marked the turning point between high levels of lead-silver production during the Roman Empire period, and much lower levels observed from the mid-second century until the mid-eighth century. The plague disrupted mining through high mortality in, and flight from, mining regions, and reduced demand through population loss.”16
It is possible that historians have made too much of the plague, as archeologists have done a sizable amount of rural survey work that demonstrates that the Empire’s population density grew steadily to a fourth-century peak. How then do we account for the difference between our narrative sources and the archeological evidence? There are likely several explanations that acted in tandem. The first is that the plague was so fast-moving and deadly that the shock of so many persons dying in so short a time period left an indelible impression for our sources to pass on. There is a very good possibility that this plague mostly afflicted those concentrated in ports, cities, and troop barracks, and may have spared most remote rural areas. Of course, these same cities that took the hardest hits are where our sources are most likely resident. Finally, Roman society was stalked by death, and as much as half the population may have died by age fifteen. Given the food surplus that went hand-in-hand with any pre-modern plague, a society trapped by Malthus’s arithmetic would have recovered very quickly. Rome, having recovered from the effects of the Antonine Plague, was soon struck by a second pestilence, the Plague of Cyprian.17 This time the plague struck when Rome was ill prepared to cope, smack in the middle of the third century. It is first attested in Alexandria, the great transport terminal and entrepôt for Indian and Chinese goods, in 249. It was in Rome by 251.18 The contemporary record, as one might expect at a time when the fabric of the Empire was being torn apart by invasion and civil war, is sparse. One of the better sources is Cyprian, bishop of Carthage, who describes it in stark terms:
These are adduced as proof of faith: that, as the strength of the body is dissolved, the bowels dissipate in a flow; that a fire that begins in the inmost depths burns up into wounds in the throat; that the intestines are shaken with continuous vomiting; that the eyes are set on fire from the force of the blood; that the infection of the deadly putrefaction cuts off the feet or other extremities of some; and that as weakness prevails through the failures and losses of the bodies, the gait is crippled or the hearing is blocked or the vision is blinded.19
When added to all of the other shocks being inflicted upon the Empire during this period, clearly this new plague could easily have provided the tipping point that would bring down the Empire. The saving grace appears to be that the barbarians invading the Empire were sickened by the plagues as badly as and possibly worse than the Romans. As related in the Historia Augusta:
The favor of heaven furthered Claudius’ success. For a great multitude, the survivors of the barbarian tribes, who had gathered in Haemimontum [Thrace], were so stricken with famine and pestilence that Claudius now scorned to conquer them further. . . . And so at length that most cruel of wars was brought to an end, and the Roman nation was freed from its terrors.20
Similarly:
During this same period the Scythians attempted to plunder in Crete and Cyprus as well, but everywhere their armies were likewise stricken with pestilence and so were defeated.21
As this book is being written, the world is experiencing a modern plague—COVID-19. This pandemic has killed millions and sickened probably hundreds of millions more. It has also closed huge swaths of the global economy and disrupted the lives of almost every living human being. Consider how terrible the COVID-19 impacts have been in just the developed world, where almost everyone has access to first-class medical treatment and governments are capable of spending trillions of dollars to alleviate the impact of economic closures. One can probably just barely imagine from this experience the impact of a disease many times more virulent than COVID-19 in a society with no serious treatment methods, no hope of a vaccine, no excess production and food capacity, and no money fountains (central banks) to keep priming the economy. Add to this the fact that barbarians and Sassanid armies were breaching the frontiers and driving deep into the Empire, along with multiple civil wars, and one can begin to understand the full impact the plague had on the Empire’s fortunes, as well as the strategic dilemmas confronting the emperors who ruled during this period.
Tacitus’s Germania, written at the end of the first century, lists dozens of tribes beyond the Rhine and Danube frontiers. But by the middle of the third century, most of these have disappeared from the record.22 In their place stands a much smaller number of tribal amalgamations, whose names dominate the sources until the fall of the Western Empire and beyond. Along the lower Rhine and across the Agri Decumates we find the Alamanni; a bit after their appearance the Franks are recorded along the upper Rhine; and along much of the Danube are the Goths, still broken into two large federations, the Tervingi and Greuthungi. Other groups that begin moving toward the Roman frontiers and swarming into the Empire in the early fifth century can already be detected right behind these larger amalgamations, including Saxons, Jutes, Angles, Vandals, Burgundians, Alans, and finally, pushing all before them, the Huns.
These confederations were more closely bonded economically and politically than the tribes had ever been. One should not, however, oversell this. The old tribes, such as the Cherusci and the Chatti, continued to exist and even had their own kings. In fact, at the Battle of Strasbourg (357), the emperor, Julian, faced seven kings and ten princes of the Alemanni. But these kings and princes were following the orders of a high king, Chnodomarius, who had mustered them for battle. In the past, a truly charismatic individual, such as Arminius—the victor at the Battle of Teutoburg Wald—could bring the tribes together for at least a short period, but other examples are rare. But starting in the third century high kings among the Alemanni and other tribal groups, particularly the Goths, became the rule rather than the exception.23 The title of high king was not a hereditary one, and the Roman sources never deigned to explain how the system worked. We can, however, assume that we see the very beginnings of a feudal system, where a high king expected lesser kings and princes to muster their armed followers on demand, as well as provide him with some type of financial or in-kind tribute.
What brought this political transformation about? Most crucially, there was an economic revolution all along the frontier that gradually spread eastward. When Julius Caesar first led his legions into Germania, the tribes were generally nomadic, as they had no way to maintain the fertility of the soil. That meant that family groupings would have to pick up and move every few years in a version of slash and burn agriculture. For the frontier tribes, one of the great benefits of living alongside the Roman Empire was the regular transfusion of knowledge, including farming techniques. By the end of the first century German farmers were settling in one place for their entire lives, as villages grew much larger and persisted in one location for generations. By the fourth century, this knowledge had moved deep inland, and there is substantial archaeological evidence of large Gothic settlements north of the Black Sea.
Settled agriculture always results in a population explosion, and there is evidence of immense population growth throughout the Empire’s frontier regions in the two centuries of the Principate. Increased food production is also the handmaiden of increasing wealth and worker specialization. So it is little wonder that we find mines capable of producing millions of pounds of raw iron deep in Poland and that burial sites continually amaze archeologists with the amount of gold and silver buried with elite personages, as well as the beautiful craftsmanship demonstrated by jewelry makers. Wealth would also have increased and become more centralized because of trading with Rome, as well as the subsidies Rome often paid tribal leaders in return for peace and support.
This economic revolution naturally led to a corresponding social revolution. One can easily imagine how much the prestige and power of a tribal leader increased when he had large amounts of Roman gold and silver to distribute among his followers, particularly if he was able to gain such subsidies regularly. The formula was simple: more food and worker specialization meant that the tribes could afford a full-time warrior class. As the increasing wealth was centralized, some tribal leaders were able to amass numbers of these specialized warriors. This permanent warrior class could then be employed to increase their leader’s sway among weaker tribes. This turned into—if one can use the term for a driver of violence and conquest—a virtuous circle, as every military victory increased the prestige and wealth of the victor and drew more chieftains and their warriors into his orbit. Once again, this is not a feudal system yet, but it is rapidly heading down that road.
These tribal amalgamations were something new in the strategic firmament. Individually, none of them could threaten the power of Rome, but they also could not be easily defeated. In the first centuries of the Empire, a well-organized assault upon a single tribe could cause substantial destruction, and Rome’s expeditionary force could return to its frontier bases secure in the knowledge that the barbarians had been cowed, likely for years to come. By the end of the second century, however, this had changed, and one has only to look at how hard-fought were the campaigns of Marcus Aurelius to get a feel for how profound such changes were. Barbarians, as was discussed earlier, could never be taken for granted, but even after the great reverse at Teutoburg Wald, Germanicus could still march to the Elbe River confident that his force would not meet its match. Marcus Aurelius, on the other hand, fought for years with practically no gains. Similarly, Maximinus Thrax, whose rise to the purple marked the beginning of the third century’s military anarchy, began his reign with a plunge into Germania. Herodian tells us that he claimed victory, but also that the Roman casualties were severe. He appears to have made no impression on the Alemanni, who were almost constantly on the attack in the following decades.
Still, the frontier defenses and the legions were sufficient to hold off even an amalgamation of tribes, something that would not be true if the Empire’s resources were stretched by multiple threats. For instance, a Sassanid attack that coincided with massive barbarian incursions would stress the Empire to the point where disaster loomed. If a civil war was added to the mix, disintegration could not be far behind. Such coordination between the Sassanids and the barbarians was never possible, and it is exceedingly unlikely that the various barbarian amalgamations ever coordinated their activities. Still, the barbarians, from long association with Rome, were attuned to what was going on within the Empire. They knew when the legions were denuding the frontiers for a war in the east, or when the Romans were distracted by civil war. They could feel the vigilance along the frontiers lessen, and they knew when it was time to roll the iron dice of war.
Time was also on the barbarians’ side. Their numbers were growing, as was their wealth. Both were driving a militarization of Germanic society, which was always going to find an outlet testing itself against the Romans. Rome was able to recover from its near-death experience in the third century, but so did its enemies. There is little evidence that the barbarians across the frontiers of Rome were even much damaged during this century. Moreover, larger and more warlike groups of Goths, Alans, Vandals, and finally Huns were approaching. This mass would push against the frontiers of the Empire until, at the start of the fifth century, they broke like a torrent upon a cracking dam.
A casual observer glancing at a map and tallying Rome’s resources at the start of the third century would be hard pressed to spot any weaknesses that may have grown since the early days of the Principate. If anything, the Empire appeared stronger and wealthier than it ever was. Mesopotamia had recently been added as a province at the expense of the Parthians; the population was recovering from the plague and may already have exceeded the numbers it boasted during Augustus’s reign. All of this was protected by the fearsome legions and a sound frontier defensive system. But Rome’s strategic approach, as well as its defenses, was built to face the challenges and threats of past centuries. Roman strategy was looking backward, and in a pattern repeated by strategists for the next two eons, they were fully prepared to fight the last war. Still, Rome had the resources to adapt on the fly. What stopped them from doing so was the internal rot of their political system. At a time when Rome needed to marshal all of its resources to confront enemies on the far side of the frontier, it was beset by civil war and political anarchy, both of which wreaked havoc on the Roman economic system. Periodically a military strongman, such as Diocletian or Constantine, would establish himself at the top, halting and for a time reversing the rot. But the center could never hold, and within a generation or two Rome was once again at war with itself. This systemic political decay accelerated the Empire’s political and economic fragmentation and was the final strategic change Rome faced in its last century. It was the failure to meet this challenge that made it impossible to successfully confront all of the others. In the end, Rome’s demise was not brought about by an accumulation of external shocks. Those could have been weathered. It was Rome’s continuous suicide attempts, as the elites that sat atop the Roman political, military, and economic systems placed their personal ambitions above the needs of the state, that caused the collapse of the Roman Empire.