

The victories that extended Rome's reach and prompted such grand processionals as the one depicted here in the Roman Forum also brought increased tensions among conquered populations along its borders. (Illustration by Francis Vyvyan Jago Arundell/Private Collection/© Christopher Wood Gallery, London/The Bridgeman Art Library)

‘Once the invaders had breached the border defenses, Rome lacked the military strength to expel them and instead settled them’

‘[I am] holding a wolf by the ears’ —Tiberius, emperor of Rome (r. 14–37)

The Roman empire was antiquity’s largest and most powerful state. It reached its zenith under Trajan (r. 98–117), encompassing nearly 2 million square miles and containing some 60 million people. Linking its provinces were more than 250,000 miles of roads, 50,000 of which were paved. Roman engineers founded or improved more than 1,000 cities and towns, transforming the rural European landscape into a marvel of urbanization. In the third century the Roman army could field 450,000 infantry and cavalry and 45,000 sailors and marines. By the time Constantinople fell to the Ottoman Turks in 1453, the Roman army—the oldest continually existing social institution in the Western world—had been on the march for two millennia.

Rome itself was a magnificent example of cultural, technological and social superiority in its time. In 356 the city had 28 libraries, 10 basilicas, 11 public baths, two amphitheaters, three theaters, two circuses (the Circus Maximus could seat 150,000 people; the Colosseum, 50,000), 19 aqueducts, 11 public squares, 1,352 fountains and 46,602 apartment buildings. Yet little more than a century later barbarian invaders stood astride the empire’s corpse, the capital in ruins.

The reasons for the empire’s demise remain among the great unsettled historical debates. Regardless, it is possible to identify some of the primary forces that rendered the imperial government incapable of dealing with the lethal challenges that beset it. Of all the factors draining the empire of its ability to survive, four stand out: the changing nature of the external threat to the empire’s western borders; the frequent civil wars among claimants to the imperial throne; the migration and settlement of large, armed and culturally hostile barbarian populations within the imperial borders; and the gradual erosion and eventual demise of the empire’s manpower and the taxpayer base required to sustain, defend and administer the Roman state.

The Romans called the area beyond the western imperial border along the Rhine and Danube rivers the land of the barbarians. Its mostly Germanic-speaking inhabitants were relatively few in number and lived in small villages, their populations limited by their primitive agricultural technology. Employing only the wooden scratch plow, German farmers could not turn the earth sufficiently to maintain its fertility. The soil’s ability to sustain adequate agricultural production quickly declined, forcing the population to move every generation or so in search of more fertile land.

The small populations and nomadism of the German tribes retarded development of their political structures. Governments were local, comprising mostly clan chiefs whose ruling power was limited by councils of advisers drawn from among other influential clan members. An individual chieftain did not have the wealth or manpower to form a warrior group loyal to him alone. Instead, clan warriors came together as circumstances required. These groups were usually small and capable only of conducting limited raids. In a few instances the tribes provided warriors to serve in limited military capacities in the Roman army itself.

The Romans used subsidies, trade, military honors and punitive expeditions to ensure the loyalty of the tribes, often playing them off against one another. The German tribes along the western border did not pose a threat to Roman garrisons, and the limited size of the tribal populations meant there was no mass migration. Indeed, the Romans occasionally settled small groups as farmers on the imperial side of the border.

But by the third century circumstances in this region had changed. The presence of Roman garrisons and merchants spurred the economic and sociopolitical development of the border tribes. The garrisons provided lucrative markets for local agricultural products, metals, slaves and military recruits. The introduction of Roman agricultural techniques—use of the deep-running iron plow, manure fertilization and irrigation—brought larger farms, abundant food, an explosion in tribal populations and the establishment of towns, all of which ended the clans’ traditional agricultural nomadism and stabilized the populations.

These larger tribes required more complex political and organizational structures. Among the more significant developments was the ability of now-wealthy tribal chiefs to support a large warrior class. By the end of the second century the populations of the tribes along the western imperial borders had grown considerably. When the chiefs of the tribes arranged themselves in confederation under the temporary authority of elected leaders, these confederations could easily field 10,000-man armies.

At the same time the discovery of large metal ore deposits beyond the imperial border created a local armaments industry. Just two deposits within the present-day borders of Poland produced 16 million pounds of iron during the Roman period. Before this find, metal weapons were produced singly, were very expensive and were strictly controlled by Roman authorities. By the third century local factories along the border were producing thousands of weapons––chiefly swords and spear points––equipping not only the Roman garrisons but also many tribal warriors. As long as the clans were content to remain under imperial control and limit themselves to periodic raiding, they were no threat to the Romans. But with pressure from other tribes seeking better land or plunder, the large border clans began to present the threat of mass migration across the imperial borders.

The tribal chieftains, who were by this time able to support and equip considerable armies, adopted the Roman practice of compulsory, full-time military service. The result was the emergence of well-armed, semiprofessional military forces whose leaders had often served in the Roman army. An example of this long-standing practice was Arminius, the German chieftain whose forces massacred three Roman legions at the AD 9 Battle of the Teutoburg Forest.

Thus, even by the end of the second century the nature and size of the threat facing the Romans across the western imperial border had changed drastically. The Germanic-speaking tribes had become large, politically organized confederations led by warrior chieftains with competent armies. They remained largely content to raid and plunder Roman settlements on their side of the border. While such raids sometimes brought harsh Roman retaliation, they just as often brought the tribes higher subsidies and greater economic opportunities. But when pushed from behind by other tribes, or drawn by the lure of a better economic life, those tribes could pose a significant threat.

In the winter of 166 this new reality burst forth when two Germanic-speaking tribes, the Langobardi and Ubii, raided the Roman province of Pannonia (present-day Hungary south of the Danube). A year later two other powerful tribes, the Marcomanni and Victuali, demanded to cross the Danube and settle within the empire. Prompting these demands was pressure from tribes beyond the frontier zone that sought to conquer the area for their own use. Rome was slow to respond. Emperor Marcus Aurelius (r. 161–180) was at war with the Parthians and had been diverting troops to that conflict. The Romans were eventually able to contain the pressure, but not before German raiders had besieged Aquileia, in Italy itself, and the conflict had dragged on for a decade.

The Marcomannic wars were no mere border raids. The German tribes sacked a number of cities and wrought widespread damage. Amid the warfare some of the tribes attempted large-scale migration. Between 235 and 275 barbarians launched major raids all along the western Roman frontier, some settling in within the imperial borders. Scholars call this period the Third Century Crisis. The warrior tribes were now a force to be reckoned with and would remain so for the next two centuries until, ultimately, Roman defenses on the Rhine and Danube frontier collapsed completely.

A major factor in Rome’s inability to deal with the growing barbarian threat was the recurring state of civil war between emperors and usurpers—the latter usually generals or other officers who supported some rival claimant to the throne. These clashes brought high casualties, disrupted training and the manpower supply, and drew units away from the Rhine and Danube frontiers. In their respective conflicts with Emperor Theodosius I, Magnus Maximus (r. 383–388) and Flavius Eugenius (392–394) so depleted the Roman border garrisons that the defense of the Rhine was dependent almost entirely on the loyalty of local barbarian client kings.

Before the reign of Marcus Aurelius the western Roman empire had experienced few serious civil wars. But between his death in 180 and the deposition of the last western emperor, Romulus Augustus, in 476, the empire witnessed more than 100 instances of armed violence as would-be usurpers challenged sitting emperors. Some of these conflicts lasted years, exhausting Roman military manpower and leaving the frontiers vulnerable. Ironically, most of the emperors and usurpers then died at the hands of their own subordinates.

The frequent civil wars ensured that fear of usurpers—not threats to the empire’s borders—became the leading concern for Roman emperors. Since no usurper could hope to succeed without the support of a substantial army, Roman administrators broke up the larger provinces to minimize the military power of any one provincial official. The breakup of the provinces began under Septimius Severus (r. 193–211), and Diocletian (r. 284–305) continued the process until some of the provincial garrisons were too small even to deal with local raiders.

Early in the fourth century Constantine (r. 306–337) drastically changed the organizational structure of the Roman army. A usurper who had gained power through civil war, Constantine was most concerned with protecting himself from other would-be emperors. He created large, mobile field armies called comitatenses, commanded by himself and intended to protect his person and thwart attempts on the throne. As part of these security arrangements he and succeeding emperors abandoned Rome as a capital, moving often to avoid threats to their safety. This crippled the empire’s central administration, as slow communications and uncertainty at the court made it difficult for the bureaucracy to operate efficiently. This was especially detrimental for the army, which relied on state bureaucrats to provide the materials, manpower and finances for war.

The comitatenses had no permanent installations and instead billeted with the civilian populations in the towns wherever the emperor held court. The old 5,000-man frontier legions were reduced in size, as were all garrison commands on the frontiers—again to keep usurpers from mustering large military units. Many frontier forts and strongpoints were reduced in size, and others simply abandoned, forcing the army to rely on local barbarian recruits to defend the frontier. Even the comitatenses armies were mostly pieced together from the remnants of the various civil war forces. By the middle of the fourth century the Roman emperors had to rely on weakened and poorly positioned military forces to deal with threats along the frontiers, internal rebellions and raids by the barbarian tribes already settled within the empire.

Despite the civil wars and the empire’s reduced military capability, Roman emperors in the fourth century were able to contain the border threats surprisingly well. New tribes continued to press settled groups closer to the border, and Roman power helped client chiefs resist the invaders. In some cases Roman administrators settled tribes of moderate size within the empire’s borders, dispersing their populations to work vacant farmland.

Far to the east, however, a large coalition of nomadic tribes led by the Huns began moving toward the Danube frontier. Recruiting warriors from conquered tribes along the way, the Hunnic army swept over the middle Hungarian plain, sending local populations fleeing toward the Roman border for protection. In the summer of 376 two groups of displaced Goths—some 15,000 warriors and 60,000 women, children and the elderly—arrived on the banks of the Danube, requesting asylum within the imperial borders and permission to settle in Thrace. Emperor Valens (r. 364–78), in order to fight Persia, had stripped the Danubian frontier of forces, and, unable to repel the Goths, he agreed to their settlement.

Things soon went horribly wrong. There was an acute shortage of food in the region, and a corrupt Roman commander named Lupicinus gathered up all the dogs in the area and sold them to the starving Goths; his going rate was one child for one dog. This set the Goths on a rampage, and they ravaged northern Thrace. Valens made peace with the Persians in 377, and the following year the Goth and Roman warriors met in battle at Adrianople. In a stunning upset the Goths killed two-thirds of the Roman troops along with the emperor. They continued fighting in Thrace until concluding a peace in 382 that allowed some of them to settle in Italy and the rest in Thrace.

The dam had burst. The success of the Goths in defeating the Roman army, in obtaining resettlement, plus the desire for a better life and Hunnic pressure spurred more tribes to cross the weakly defended border. The defeat at Adrianople had destroyed 60 percent of the Roman army of the east. The Romans tried hard to stop what had become a mass migration, attacking the migrants as they attempted to cross the rivers and hunting them down as they moved inland.

Between 405 and 408, during a period historians refer to as the Fourth Century Crisis, the empire suffered other large-scale barbarian invasions. Roman losses during the campaigns between 395 and 410 were horrific; some estimates claim the invaders shattered as many as 80 regiments—nearly 50 percent of the Roman field army in the west. Short on troops, desperate Roman commanders resorted to hiring the warriors of tribes already settled in the empire. Barbarians now fought to keep other barbarians from entering the empire.

Once the invaders had breached the border defenses, Rome lacked the military strength to expel them and instead settled them in various provinces, on the condition they provide troops to the Roman army. But settling the migrants did not end the problem. The barbarian settlements, with their own rulers and strong armies, resisted Roman efforts to control them. Within a few years the barbarian kings took to fighting one another and to raiding and occupying neighboring Roman settlements. Most of the empire’s interior cities and towns lacked defensive walls, a consequence of the long Roman peace, so the barbarian raids and Roman counterattacks devastated several of the provinces. The barbarian settlements became the prototypes of the feudal kingdoms that would emerge later.

With the barbarian settlement and outright occupation of some of the provinces, the flow of tax money to the imperial capital dried up. By the end of the third century an estimated two-thirds of the empire’s tax revenue no longer reached the imperial administration. The Vandals’ capture of Rome’s Spanish silver mines in 411 and the conquest of the empire’s North African provinces in 435 and 439 robbed Rome of her richest provinces in terms of grain supply and tax revenue. These events crippled imperial finances beyond repair.

Without sufficient funds, what remained of the imperial administration could no longer raise sufficient numbers of troops or train them adequately to meet the empire’s needs. In short order the once-matchless Roman army was reduced to a coterie of barbarian war bands serving under their own chiefs. The empire of Rome was now in its death throes.

The collapse of the western Roman empire throws into sharp relief the success of the eastern empire’s ability to function as an important state for another 1,000 years. For the most part the circumstances and events that brought down the western empire did not confront the eastern empire.

Geography was an important reason for the eastern empire’s survival. In the west the only natural obstacles to invasion were the Danube and Rhine rivers. In the east the main geographic barrier was the Bosporus. To cross that strait in any force required ships and the power to confront the formidable Roman navy—resources that barbarian tribes lacked. The imperial capital at Constantinople was protected in the northeast by mountains and easy to defend passes. Roman diplomacy maintained good relations with the mountain tribes that provided manpower and early warning against invasion. To the south and east the Parthians and later the Sassanid Persians blocked Arab invasions. Although the Romans had security issues with the Persians, dealing with an organized state was much easier than dealing with a number of powerful tribes attacking along an extensive and vulnerable border.

While the eastern empire was not entirely free of threats posed by usurpers, the few civil conflicts there were were short-lived and mostly ended in victory for the imperial rulers. The eastern emperors were rarely assassinated, and Constantinople remained the administrative, political and military capital, effectively controlling the army and imperial bureaucracy and finances. The stability of the capital also contributed to a persistent sense of national identity with the old empire, an identity that had been lost in the west. To the very end inhabitants of the eastern realm thought of themselves as Romans.

The geography that made invasion of the eastern empire difficult had the effect of diverting the invaders to the west, where it was easier to assault the imperial frontier. The geographical conditions and the still-effective Roman army kept migrating tribes from breaking through the eastern defenses in large numbers. So the eastern emperors were never forced to settle large barbarian populations with independent armies capable of threatening the empire from within. Moreover, the Roman army of the east deliberately limited the number of barbarian soldiers allowed in its ranks.

After the collapse of the western Roman empire, the eastern—Byzantine—empire withstood the efforts of various attackers until 1453, when Constantinople came under full-scale assault by the Ottoman Turks, armed with some of the earliest siege cannons in history. Although the Roman army faced certain death, perhaps mindful of its noble heritage, it put up one hell of a fight.

Richard A. Gabriel, a distinguished professor of history and war studies at the Royal Military College of Canada, is the author of more than 40 books. For further reading he recommends Empires and Barbarians: The Fall of Rome and the Birth of Europe, by Peter Heather, and How Rome Fell: Death of a Superpower, by Adrian Goldsworthy.