The so-called Third Century crisis brought the Roman Empire to the brink of destruction. Only through the efforts of several capable soldier emperors, Rome not only recovered but was able to remain a great power for another century. The late Roman Empire, however, was a different beast than its earlier iteration. The rule of one monarch was replaced by two or more co-emperors. The division of power facilitated the government over the vast territory, enabled easier responses to emerging crises, and lessened the potential for usurpation. The army, too, was reformed, resulting in a large number of smaller but more mobile rapid-response elite units (field armies), the comitatenses, paired with the lower-quality limitanei who patrolled the frontier. In addition, the military needs dictated the shift of the imperial center from the West to the East, to the new capital Constantinople.
The increased pressure on the empire’s frontiers, especially in the East, and a series of civil wars, weakened the imperial military capabilities. Nevertheless, the eastern part of the late Roman Empire managed to survive, and after dealing with several crises, continued to thrive. The Roman West, however, buckled down under pressure and fell apart in the late fifth century.
1. Battle of Milvian Bridge (312 CE): The Beginning of the Christian Roman Empire
Diocletian’s voluntary abdication in 305 CE brought his experiment to an end. The Tetrarchy—the joint rule of four emperors, two senior (augusti) and two junior (caesares)—collapsed in blood. Ironically, the men who toppled the Tetrarchy were the sons of former tetrarchs in the West, Constantine and Maxentius. Constantine enjoyed the support of the army in Britain, while Rome supported Maxentius. Tetrarchy was not based on blood but merit. Nevertheless, the two ambitious men decided to lay their claim, plunging the late Roman Empire into civil war. After the reigning augusti, Galerius and Severus (the latter perished in the struggle), failed to defeat Maxentius in the spring of 312 CE, Constantine (now in control of Britain, Gaul, and Spain) marched on Rome.
Constantine’s legions quickly overran north Italy, winning two major battles at Turin and Verona. In late October, Constantine reached Rome. The emperor, allegedly inspired by a vision from God in the sky – “In hoc signo vinces” (“In this sign, you shall conquer”) – ordered his soldiers to paint the heavenly sign on their shields. This was probably the Chi-Rho (☧) sign, marking Christ’s name, later used on the military standards. The “heavenly vision” could be a solar halo phenomenon, fitting well into Constantine’s belief in the solar deity – Sol Invictus – popularized by his predecessors, most notably soldier-emperor Aurelian. Whatever transpired the night before the battle, the following day, Constantine led his troops to victory.
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Instead of remaining in the safety of Rome’s imposing walls, Maxentius set out to meet the attackers in the open battle. He had already ordered the destruction of the Milvian bridge, one of the main access routes to the ancient city. So, Maxentius’ men crossed the Tiber over the improvised wooden or pontoon bridge. It was a grave mistake.
On the 28th of October, the two armies clashed in front of the now-razed Milvian Bridge. Maxentius drew his battle line with the Tiber too close to its rear, limiting the mobility of his troops in the case of retreat. When Constantine’s cavalry charged, followed by the heavy infantry, Maxentius’ men, who up to that point offered stiff resistance, received the order to retreat. The usurper probably wanted to regroup within the city, drawing the enemy soldiers into the costly urban warfare. Yet, the only way to retreat was a flimsy temporary bridge. Under the attack of Constantine’s crack troops, the withdrawal soon turned into a rout and the bridge collapsed. Most of Maxentius’ soldiers, including the hapless emperor, drowned in the river.
Maxentius’ death left Constantine in command of Rome and Italy. The day after the battle, the victor entered the ancient city. Soon, Africa too recognized his rule. Constantine was now the master of the Roman West. The emperor pardoned the enemy’s soldiers, but with one exception. The Praetorian Guard, which for centuries acted as a kingmaker, was severely punished for their support of Maxentius. Castra Praetoria, their famed bastion that dominated Rome’s cityscape, was dismantled, and the unit was disbanded for good. Another elite unit, the Imperial Horse Guard, followed the same fate, being replaced with Scholae Palatinae. The grandiose Arch of Constantine still stands in the center of Rome as a witness of the epochal victory.
Constantine took an active interest in promoting and regulating the Christian religion. Still, he himself converted to Christianity only on his deathbed in 337. A year after the Battle of Milvian Bridge, the emperor made a fateful decision, which would have far-reaching consequences for the late Roman Empire, and global history. With the Edict of Milan, Christianity became an officially recognized religion, paving the way for the Christianization of the Empire, Europe, and eventually, the world. A decade of civil wars followed, until in 324, Constantine the Great became the sole ruler of the Roman world.
2. Battle of Strasbourg (357 CE): The Victory That Saved Roman Gaul
Constantine the Great reshaped the late Roman Empire in more ways than one. He promoted Christianity, reorganized the imperial administration, economy, and the military, and moved the capital of the Empire to the East, naming the newly founded city Constantinople after himself. Then, as the sole ruler, he established a new dynasty, the Constantinian, leaving the Empire to his three sons. His heirs, however, followed their father’s example, plunging the Empire into yet another civil war. Realizing that he cannot rule over the vast territory alone, the last surviving son of Constantine, emperor Constantius II, appointed his only male relative, the 24-year-old Julian, as his co-emperor. Then, in 356 CE, he sent the young caesar to the West.
Julian’s task was to restore imperial control in Gaul. His mission was all but easy. The four-year-long civil war wiped out most of the Gallic army, most notably the bloodbath of the Battle of Mursa. The weak and poorly manned frontier defenses on the Rhine presented no obstacle to the Alamanni, a confederation of Germanic tribes, who crossed the great river and plundered the region. Roman defenses were in such a sorry state that the barbarians managed to capture almost all the Rhine’s fortified cities! Not willing to leave anything to chance, Constantius appointed his most trusted general, Barbatio, to supervise his young relative. Perhaps, the emperor had hoped that Julian would fail in his mission, thus lowering his chances to usurp the throne.
Julian, however, proved to be an effective military leader. For two years, the caesar fought the Alamanni and their allies, the Franks, restoring the Gallic defenses and reclaiming lost lands and towns. In addition, he managed to make peace with the Franks, depriving the Alamanni of their close ally. In 357, the large force of Alamanni and their allies, under the king Chnodomar, crossed the Rhine and seized the area around the ruined Roman fort of Argentoratum (present-day Strasbourg). Taking the opportunity, the Romans decided to crush the invaders in a twin-pronged assault. A large army of 25,000 under Barbatio was to march against the invaders, while Julian would attack with his Gallic troops. However, before the battle, Barbatio withdrew his army without informing Julian. The reasons for such an action are unclear. Julian was now left in command of only 13,000 men, with the Alamanni outnumbering him three to one.
The Germans had greater numbers, but Julian’s troops were of better quality, containing some of the best regiments in the late Roman army. They were fierce and reliable men, many of them of barbarian origin. He also had around 3,000 cavalrymen under his command, including 1,000 kataphraktoi, imposing heavily armored cavalry. Marching rapidly to seize the high ground overlooking the river, Julian arrayed his forces so the barbarians would have to attack uphill, putting them at a disadvantage.
Initially, the battle went badly for the Romans. Julian’s heavy cavalry almost bolted when Alamanni light infantry got among them, stabbing the horses’ unprotected bellies from concealed positions in the standing grain. Without the horse’s armored protection, their riders became easy prey for the barbarian warriors. Encouraged by their success, the Germanic infantry advanced, charging at the Roman shield wall. Julian himself jumped into the fray, riding across with his 200-man bodyguard, scolding and encouraging his soldiers. While costly, the barbarian attack succeeded, punching a hole through the center of the Roman frontline. Despite being cut in two, the Roman line held fast, thanks to the experienced legionaries holding the formation. The sustained attacks tired the Alamanni. It was the moment the Romans were waiting for. Moving into the counterattack, the Romans and their auxiliaries (many of which were also Germanic tribesmen) put the Alamanni to flight, pushing them into the Rhine. Many drowned, struck by the Roman missiles or weighed down by their armor.
Around 6,000 Germans died on the battlefield. Thousands more drowned while trying to reach the safety of the opposite riverbank. The majority, however, escaped, including their leader, Chnodomar. The Romans lost just 243 men. Chnodomar was soon captured and sent to a prison camp where he died of disease. The safety of Gaul was restored once again, with Romans crossing the river in a brutal punitive campaign. Julian, who was already popular among the troops, was acclaimed as augustus by his troops, an honor he refused, as only Constantius could legally bestow the title. However, in 360, when his eastern colleague requested Gallic legions for the Persian campaign, Julian refused the order and accepted the will of his troops. Constantius’ sudden death spared the late Roman Empire from a civil war, leaving Julian its sole ruler.
3. Battle of Ctesiphon (363 CE): Julian’s Gamble in the Desert
In 361 CE, following the death of Constantius II, Julian became the sole ruler of the late Roman Empire. He, however, inherited a deeply divided army. Despite his victories in the West, the eastern legions and their commanders were still loyal to the late emperor. To overcome the dangerous division and decrease the potential for a revolt, Julian decided to invade Persia, Rome’s main rival. The goal was Ctesiphon, the Sassanid capital. The triumph in the East, long sought by Rome’s leaders, and achieved only by a few, could also help Julian pacify his subjects. In the rapidly Christianizing late Roman Empire, the emperor was a staunch pagan known as Julian the Apostate. In addition, by defeating the Sassanids on their home turf, Rome could stop hostile raids, stabilize the frontier, and perhaps get further territorial concessions from his problematic neighbors. Lastly, a decisive victory could provide an opportunity to install an imperial candidate on the Sassanid throne.
True, the lure of the East spelled doom for many would-be conquerors. Julian, however, held all the winning cards. At the emperor’s command was a large and powerful army composed of both western and eastern legions and led by veteran officers. Julian’s ally, the Kingdom of Armenia, threatened the Sassanids from the North. Meanwhile, his enemy, the Sassanid ruler Shapur II was still recovering from a recent war.
Julian entered the Persian territory in March 363. After Carrhae, where centuries earlier Crassus had lost his life, Julian’s army split into two. A smaller force (around 16,000-30,000) moved towards Tigris, planning to join the Armenian troops for a diversionary attack from the North. The emperor, leading more than 60,000 troops, advanced down the Euphrates, accompanied by more than 1,000 supply boats and several warships. Taking one Sassanid fort after another and razing them to the ground, the Roman army quickly reached Tigris, restoring Trajan’s canal and transferring the fleet.
In late May, the legions approached Ctesiphon. To avoid a protracted war in the sweltering heat of Mesopotamia, Julian decided to strike directly at the Sassanid capital. Following a daring night attack across the river, the legionaries landed on the other bank, overcoming the resistance, securing the beach, and pressing forward. The Battle of Ctesiphon unfolded on a broad plain in front of the city walls. The Sassanid army, arrayed in the typical fashion, with heavy infantry in the middle, flanked by light foot and heavy cavalry including several war elephants. The Persian commander planned to soften the Roman heavy infantry with the signature hail of arrows and then break the hostile formation with the terrifying charge elephants and mail-clad clibanarii.
However, the Sassanid attack failed. As the Roman army was well prepared and had good morale, it offered strong resistance. Julian also played a significant role, riding through the friendly lines, reinforcing weak points, praising brave soldiers, and castigating the fearful. Once the Persian cavalry and elephants were driven from the battlefield, the entire enemy line buckled, giving way to the Romans. The Persians retreated behind the city gates, leaving more than two thousand dead. The Romans lost only 70 men.
Although Julian won the battle, his gamble failed. Unable to take Ctesiphon by force, or provoke the decisive battle, Julian and his commanders were left with a difficult decision. Should they confront the approaching main force under king Shapur II, risk it all, or withdraw? The emperor opted for the latter. He ordered all the ships to be burned and withdrew westwards. The retreat, however, was slow and arduous. The sweltering summer heat exhausted the Roman troops, while the hit-and-run attacks by Persian mounted bowmen weakened the soldiers’ morale. Several days later, on 26th June 363, emperor Julian lost his life in the enemy’s attack. Deprived of their leader and unable to mount an efficient defense, the Roman army capitulated, agreeing to a humiliating peace in exchange for safe passage to the border. Instead of the triumph, the late Roman Empire suffered a disaster, with Ctesiphon forever remaining out of imperial reach.
4. Battle of Adrianople (378 CE): Humiliation and Disaster
Julian’s sudden death left the late Roman Empire in disarray. The imperial army was humiliated and leaderless. To make matters worse, his successor—emperor Jovian—died before reaching Constantinople. Faced with the possibility of another civil war, the commanders of both field armies elected a compromise candidate. Valentinian I was a former officer who would prove to be an excellent choice. His reign would bring stability and prosperity to the Roman West. His co-emperor and brother, the eastern emperor Valens, would not fare so well, nearly losing the throne at the very start of his reign. Furthermore, the threat from the East loomed on the horizon. Thus, when in 376 CE the Gothic tribes asked the Roman authorities for permission to cross the Danube, as they fled from the Huns, Valens was only too happy to agree. The fierce warriors could fill the depleted ranks of his legions, bolster the frontier defenses, and strengthen the Eastern Empire as a whole.
While Valens’ plan was sound, the settlement of the Goths would soon turn into Rome’s nightmare. The large influx of barbarians led to friction with the local authorities. After being mistreated and humiliated, the Goths went to war with Romans. For two years, the Thervingi under Fritigern and the Greuthungi under Alatheus and Saphrax rampaged through Thrace, joined by bands of Sarmatians, Alans, and even Huns. Instead of stability, Valens reaped chaos. By 378, it became clear that the barbarian threat must be eliminated in one direct strike. Hearing that the Goths had established the camp in the vicinity of Adrianople, Valens transferred all the forces from the Eastern frontier and took the army’s leadership.
Valens marched the eastern field army out of Constantinople to attack the Goths without waiting for reinforcements from the western emperor Gratian. Soon his scouts notified him of a smaller force (around 10,000) led by Fritigern. Valens was certain that he would score an easy victory. Unfortunately, the reconnaissance had failed to spot the barbarian cavalry led by Alatheus and Saphrax, which were away on the raid. Thus, the emperor dismissed Fritigern’s envoys and prepared for battle.
In the early afternoon, the Roman troops came within sight of the Gothic camp, a circle of wagons protected by the ditch and the palisade. Fritigern once again called for a parlay, which Valens accepted. His men were tired and thirsty from marching under the hot summer sun and were not in battle formation. As negotiations were starting, however, fighting broke out between the two sides. Valens ordered a general attack, even though his infantry were not fully prepared.
At this point, the Gothic cavalry returned, descending upon the Romans from the hill. The enemy charged the Roman right flank, routing the cavalry, which left the infantry exposed to the attack from the rear. At the same time, Fritigern’s warriors emerged behind the wagons to strike the legionaries from the front. Surrounded and unable to break out, tightly packed Roman soldiers were slaughtered by the tens of thousands.
The defeat at Adrianople was likened by the Roman historian Ammianus Marcellinus to the second worst disaster after Cannae. Around 40,000 Romans, two-thirds of the eastern field army, laid dead on the battlefield. Most of the eastern high command had been slain, including emperor Valens, who perished in the fighting. His body was never found. Less than two decades after Julian’s death, the throne in Constantinople was vacant once again. This time, however, the late Roman Empire faced grave peril. Invigorated by the incredible victory, the Goths ravaged the Balkans for several years until the new eastern emperor, Theodosius I, made a peace settlement. This allowed the barbarians to settle on Roman soil, this time as unified people. Theodosius’ decision would have a fateful consequence for the late Roman Empire and play a role in the emergence of the barbarian kingdoms.
5. Battle of Frigidus (394 CE): The Late Roman Empire’s Turning Point
Following the disaster at Adrianople in 378 CE, the Western Roman emperor Gratian appointed general Theodosius as his co-ruler in the East. While he was not a member of the ruling dynasty, Theodosius’ military credentials made him an ideal choice for restoring imperial control over the Balkans which were under Gothic attack. In 379, the eastern emperor fulfilled his task, reaching a peace settlement with the barbarians. Yet, while Theodosius brought to an end the year-long crisis, he would also play a major role in the weakening and eventual loss of the Roman West.
Unlike the previous agreements with the barbarians, the Goths were settled as unified groups and served in the Roman military under their own commanders, as the foederati. More importantly, the ambitious Theodosius had plans for his own dynasty. Following Gratian’s demise in the civil war, the eastern emperor acted as his avenger, defeating the usurper Magnus Maximus in 388. Only four years later, in 392, Gratian’s younger brother and western Roman emperor Valentinian II died in mysterious circumstances. Arbogast, the powerful general with whom the young emperor repeatedly clashed, was declared a culprit.
Arbogast was Theodosius’ former general and right-hand man, whom the emperor personally dispatched to be Valentinian’s guardian. With his powers significantly limited, it is likely that hapless Valentinian was not killed but committed suicide. However, Theodosius rejected Arbogast’s version of events. In addition, he did not recognize Arbogast’s choice for the emperor; Flavius Eugenius, a teacher of rhetoric. Instead, Theodosius declared war on his former ally and presented himself as Valentinian’s avenger. However, he was already planning the establishment of the new dynasty, clearing the path to the throne for one of his two sons. In 394, Theodosius marched with an army onto Italy.
The opposing armies were equal in strength, numbering around 50 000 men each. The eastern field army, however, was still recovering from the losses experienced less than a decade ago. Its ranks were bolstered by 20,000 Goths under the command of their leader Alaric. The two armies met in present-day Slovenia, by the Frigidus river (most probably Vipava). The narrow terrain, surrounded by high mountains, limited the army maneuverability and tactical options. Theodosius had no other choice than to commit his forces to a frontal attack. It was a costly decision. Alaric’s Goths, who formed the bulk of attacking troops, lost almost half of their forces. It seemed that Theodosius would lose the fight. However, on the following day – bora – a particularly strong gale wind blew from the east, blinding the enemy with dust, almost knocking down the Western troops. It is likely that the sources employed some poetic license, but even today, the Vipava valley is known for its strong winds. Thus, the force of nature helped Theodosius’s troops to win a total victory.
The victor has shown no mercy to hapless Eugenius, beheading the usurper. Arbogast, deprived of his forces, fell on his sword. Theodosius was now the sole master of the late Roman Empire. His rule, however, did not last for long. In 394, the emperor died, leaving the Empire to his two sons, Arcadius and Honorius. Theodosius’ achieved his aim, establishing his own dynasty. Traditionally, the Battle of Frigidus is remembered as a clash between last vestiges of paganism and rising Christianity. However, there is no evidence that Eugenius or Arbogast were pagans. The accusations could be a product of Theodosius’ propaganda, aiming to enhance the emperor’s victory and legitimacy. Yet, the costly victory at Frigidus had another lasting impact on the late Roman Empire, particularly the Western half.
The losses at Frigidus decimated the western field army, lowering the defensive capabilities of the Roman West, at the moment when the barbarian pressure on its frontiers had increased. In addition, Theodosius’ sudden death (he was 48) left the western throne in the hands of his underage son, who had no military experience. While the strong bureaucracy in Constantinople kept his brother Arcadius (and his immediate successors) in firm control of the eastern Empire, the Roman West came under the control of the strong military men with no dynastic background. The infighting between the powerful generals, and the recurrent civil wars, further weakened the army, allowing the barbarians to take over parts of the Roman West as the fifth century progressed. By 451, the western field army was in such a sorry state that its commander Aetius had to negotiate an uneasy alliance with the barbarians, to stop the Huns at Chalons. Finally, in 476, the last western emperor (a puppet), was deposed, bringing the end to Roman rule in the West.