Even Augustus could not manage the affairs of the entire Empire alone, particularly in times of crisis. In the early years of the Empire he could rely on Agrippa to enforce his will, and when Agrippa passed there was Tiberius. By the third century one-man rule was clearly unable to deal with the multiple simultaneous threats facing the Empire. A succession of emperors had tried various solutions, with little lasting success. It was Diocletian’s genius to recognize the political truth and attempt to institutionalize a new ruling system—the tetrarchy. This system, based on two Augusti—one for the east and one for the west—each served by a Caesar, aimed to ensure that the man at the point of any crisis had the power to act and that he would be obeyed.1 Just as crucially, it was meant to regularize the succession and thus end the debilitating civil wars of the past: when an Augusti died or retired, his Caesar would replace him and a new Caesar, acceptable to all, would be invested. The system worked as long as Diocletian was alive, for his stature and demonstrated ruthlessness was sufficient to keep everyone in line. But when he passed from the scene, his arrangements proved no match for human nature and dynastic ambition.
From the start, Diocletian recognized the system’s weakness and tried to alleviate it through a series of marriage arrangements: Constantius married Maximian’s daughter, while Galerius married Diocletian’s daughter, and both Caesars were adopted by their respective senior Augusti. Later, Maximian’s son, Maxentius, married Galerius’s daughter, and Constantius never forgot about Constantine, his son from an earlier marriage to Helena. Because neither Diocletian nor Galerius had sons, Maxentius and Constantine certainly considered themselves to be next in line as Caesars, followed by becoming Augusti when the first generation of the tetrarchy passed on. When their expectations, particularly those of Constantine, were not met, they proved only too willing to blow up Diocletian’s arrangements in favor of seeking a battlefield decision.2
While historians have made much of Diocletian’s political arrangements, their lack of staying power makes them virtually a strategic nullity. Diocletian could have appointed all three of the other members of the tetrarchy simply to military commands and achieved the same results. His one true achievement was making sure that his system managed to keep the ambitions of his colleagues in check during his lifetime. But because that collegiality broke down immediately after his death, one can hardly call it a lasting impact. Still, from the start of Diocletian’s reign until the fall of the Western Empire, there were very few years in which the Empire was not divided between multiple rulers, and most of those years are accounted for by Constantine’s sole rule.3 The problem, of course, was that these co-rulership schemes almost always degenerated into renewed bouts of civil war.
Diocletian’s other major reform was to reconfigure the provinces. By far the most crucial changes were a doubling of the number of provinces to approximately a hundred, and then the grouping of adjoining provinces into one of a dozen dioceses, which were then all tied to Diocletian’s new fiscal and administrative arrangements. Each of the new dioceses—six in the west (Britanniae, Galliae, Viennensis, Hispaniae, Africa, and Italia), three in Illyricum (Pannoniae, Moesiae, and Thracia), and three in the east (Asiana, Pontica, and Oriens)—was placed under an equestrian official, called the vicarius of the praetorian prefects.4 In most cases, the vicarii had no control of the military forces stationed within the entire dioceses. The command of the military forces within a diocese fell to a comes rei militaris, who reported to the magister militum, who commanded the armies of each of the Augusti. The comes controlled the duces, who held military command within the various provinces. This left the vicarii to manage the civilian administration of their dioceses, including tax collections and paying the soldiers. The vicarii, except for the proconsuls in the east (facing the Persians) and in North Africa (supervising Rome’s food supply), supervised the governors. As the centers for tax collection, the various dioceses soon became the later Empire’s great fiscal centers.5
Diocletian also unquestionably increased the size of both the army and the fleet, although there is no reason to entertain Lactantius’s commentary about each member of the tetrarchy recruiting a force equal to any single emperor, which would entail a quadrupling of Rome’s military forces. What is more certain is that he added to the number of legions while also subdividing them, a process begun much earlier when generals began taking vexillations rather than entire legions on campaign. As the number of legions increased, they also became smaller. This would have increased flexibility, but likely at the cost of shock, which was always a function of mass. It would also have greatly depleted the staying power of the legions on defense. The comitatus was retained, and there were certainly several of these mobile field armies at any one time.
A new office of duces (future dukes) began to appear along the frontier amid the Crisis of the Third Century. These duces were military commanders who initially fell under the command of the provincial governors. Later in the century there is a record of some duces commanding smaller field armies, usually vexillations taken from the comitatus to fight in other threatened theaters. Diocletian expanded the military authority of the duces as part of his program to separate military and civil hierarchies within many provinces. By separating the civil authority from the military power in each province, Diocletian limited the possibility of any single commander becoming powerful enough to revolt: a dux might have the soldiers, but the governor would have money. Unless both parties were ready to revolt and one agreed to remain subordinate to the other—an unlikely event—it would be difficult for any rebellion to gain traction. None of this happened overnight, and there are instances of governors still possessing military commands well into Constantine’s reign.6 At the start of the fifth century, the Notitia Dignitatum records two duces in Britain, twelve along the length of the Rhine and Danube, eight in the east, and seven in Africa.
Diocletian also placed considerable emphasis on rebuilding and strengthening the limes, especially along the eastern frontier, where the fortress cities, such as Nisibis, were greatly strengthened and new forts along known avenues of approach multiplied. Much has been made of this apparent attempt to renew the limes’ function as a preclusive barrier capable of repelling all but the most determined assaults.7 This, as we have seen, fundamentally misinterprets the reasons for Rome’s frontier fortifications. The limes could certainly deal with the management of low-level threats and could deter barbarian chiefs without a large force from attacking even when Rome was distracted by other events. But by the end of the third century, no Roman military leader could have failed to note that the barbarian tribal coalitions were powerful enough to penetrate the limes at any point of their choosing. If this is the case, then what was Diocletian’s intent? By strengthening the limes, the emperor was only partially returning to the strategic status quo, allowing Rome to benefit from the same factors that made the limes important in the first place. For one thing, frontier fortifications greatly increased security along long stretches of the Rhine and Danube, allowing the riverine fleets to move uncontested along their length. This, as it had done during the Principate, vastly improved operational and strategic mobility, since reinforcements and supplies could move quickly to any trouble spot. For another, the limes returned security to most of the frontier provinces, as they were now capable of holding back all but the largest raids. Moreover, more stable frontiers made trade across the frontier zones possible again and also allowed a more controlled migration into the Empire. Finally—and this is where Diocletian’s defensive system differed from that of the Principate—there were at least two and more likely several field armies stationed behind the limes. These armies could move to engage larger incursions without having to draw troops away from the frontiers, which would encourage further raiding. And, in the event of a truly serious threat, these armies could combine their combat power, again without necessarily having to draw troops from the frontier. Strategic success relied on finding the balance. How strong could you make the limes before you were using up troops needed to man the field armies, and how strong could you make the field armies before the frontiers became dangerously denuded of troops? Finally, as always, the great imponderable was another outbreak of civil war. If the field armies turned on one another, all such balancing calculations would be for naught.
It is worth noting that, with the field armies in place, Diocletian was no longer putting his faith in Luttwak’s preclusive strategy, which called for stopping all threats at or as close to the frontier as possible. The new system, of course, remained preclusive in the face of low-level threats. But in the face of a large-scale assault, the job of the forces in the frontier zone was to warn, delay, and channel the threat toward the fast-advancing field army. Moreover, because these field armies operated as close to the frontier as possible, they could still draw supplies from bases along Europe’s two great rivers.
What is missing from Diocletian’s system is the integration of offensive maneuver. In the past, Rome had not been content to rely solely upon the limes to meet whatever threat arose. Rather, when Rome received intelligence of dangerous concentrations of barbarians across the frontier, large punitive expeditions would plunge deep into barbarian territory to short-circuit threats before they could metastasize. Diocletian was returning stability, which was crucial to the Empire’s recovery, but he was still ceding the initiative to Rome’s enemies.
To pay for enlarging the military and rebuilding the limes, Diocletian spent considerable time and energy reforming the Empire’s tax system.8 The demographic collapse, a result of the prior century’s wars, famines, and plagues, was destructive enough to Rome’s finances, but Rome had also seen a huge amount of farmland turn fallow as a result of the wars, a problem that was multiplied by depopulation. The consequence was a major loss in revenue from Rome’s two most crucial revenue streams, the poll tax and the land tax. In many cases, the cost of maintaining the armies forced Rome to abandon taxes based on currency collection in favor of taxes in kind, which provided wheat and other necessities directly to the army.
Under Diocletian, internal stability also stabilized the army. For the first time since the Severan dynasty, the size of the army was fixed and known. Because military expenses absorbed most of the state budget, it was now possible to predict almost precisely how much revenue was required every year. Add this predictability to Diocletian’s enhanced administrative system, and for the first time Rome was able to carefully budget its expenses. At the same time, the reorganization of the Empire’s provinces, the new power of local administrators, and the transition from tax farming to a system where taxes were collected by imperial administrators increased the amount of revenue going to the imperial treasury, but also made the annual tax take much more predictable. For perhaps the first time in its history, Rome was able to calculate both revenues and expenses and create a workable state budget. This new tax system and a realignment of the tax burdens was all accomplished through a new imperial census and land survey of the entire Empire. It was also aided by Diocletian’s removal of almost all of the numerous tax exemptions that had been emplaced over the centuries.
Because most of the tax proceeds were paying for the army’s upkeep, Diocletian also regularized the substitution of gold and silver for in-kind contributions, making taxes fairer and more proportionate to agricultural yield as well as to the number of taxpayers. Moreover, because the new system accepted most taxes as in-kind payments and made these substitutes a considerable part of every soldier’s pay, both the army and the imperial administration were shielded from the consequences of currency depreciation and price rises.9 While such methods appeared to be an answer to Rome’s financial chaos, in the long run the demonetization of the economy continued to inflict damage on interregional trade, accelerating the de facto economic fragmentation of the Empire. Moreover, soldiers who were no longer drawing their pay in coinage from a central authority would inevitably start to see themselves as regional forces dedicated to the defense of a region (and its food supplies) rather than of the Empire as a whole. Still, there is a real limit to how much of the soldiers’ wages could be paid in kind. Soldiers expected to be paid in coin, and when it was not received in acceptable amounts, they tended to become dangerous.
This is what made the persistent Aurelian inflation so dangerous in the short term. When initial attempts at recoinage and establishing new values for existing coins failed, Diocletian in 301 issued the Edict of Maximum Prices. The edict listed maximum prices of food commodities as well as a long list of manufactures. It also set the pay for most laborers as well as transport and freight prices. As with every government price-fixing attempt in history, the edict was doomed to failure, as producers withheld food and other goods from the market. In a very short time a vibrant black market was thriving despite the edict’s decree of the death penalty for anyone who violated it. In time the edict was allowed to fade away unenforced as prices continued to rise.
Monetary stability and the end of inflation had to wait until the end of the civil war that brought Constantine to power in 324. By allowing the price of gold to rise while issuing large amounts of new gold coins, as had his successors, Constantine stabilized the economy by moving it from a silver base to one of gold. Constantine’s new gold coin—the solidus, weighing 1/72 of a pound—became the basis of a new and more stable monetary system, exactly as the silver denarius had done during the Principate. This switch was greatly assisted by the discovery of rich new sources of gold in the Caucasus. This new influx of bullion, once processed into high-quality solidi, first financed Constantine’s imperial ambitions and then funded his successors for decades. Moreover, this huge influx of bullion, which proved much more resistant to inflation than silver, drove significant economic growth throughout the Eastern Empire.10 Thus, by the end of the fourth century the Eastern Empire was far wealthier than it had been at the end of the Crisis of the Third Century. The centralized control and distribution of new gold resources knitted the Eastern Empire’s economy together, keeping local elites—a newly risen administrative autocracy—integrated and loyal to the imperial power that was driving the economic engine. This economic and political consolidation accelerated even as the Western elites were increasingly distancing themselves from the imperial center.11 Interestingly, very little of this gold made its way into the Western Empire, providing further evidence that the Empire was divided not only politically but economically.
While this change stabilized the monetary system for the government, the rich, and the imperial elites, it was a disaster for the mass of the population and the long-term health of the economy. Without access to gold, and with the economy bifurcating between those who could purchase goods in gold and those still trying to make do with silver and copper, the latter soon found their money even more worthless than before as inflation galloped along in the first decades of the fourth century. Economic fragmentation continued apace, which always left political fragmentation lurking in the shadows, waiting for an opportunity.12
Before turning to the end of Diocletian’s rule, we must examine the strategic impact of his edict to restart the persecution of the Christians. While the event has served to tarnish Diocletian’s place in history—most surviving histories of the period and later were written by Christian authors—it lasted only eighteen months. And although his successor, Galerius, started the persecutions anew, they were not as widespread as later Christian authors would have us believe, since many local administrators did the bare minimum required to appease the central power. In fact, Constantius, in the areas he controlled, ignored the edict completely, a position that probably did much to guide the young Constantine. His superior Augustus, Maximian, was also not an enthusiastic supporter of the persecutions. These early differences in how Christianity was tolerated or accepted are where we have to look for the persecutions’ strategic impact. In the east, the persecutions often continued apace, driving Christianity underground and accelerating the various schisms already forming within the new religion. In the west, however, Christianity was flourishing and was much more apostolic and unified in its outlook and beliefs. It is out of this milieu that the Empire gets its first Christian emperor, Constantine, which, in turn, led to Christianity becoming the dominant state religion within a remarkably short time.
By 304 Diocletian had restored Roman power to a height not seen since the reign of Severus. Having done so, he decided it was time to move aside while he still had the power and unbounded prestige to choose his successors. In 305 he asked Maximian to meet him in Rome to celebrate their twentieth year together as co-Augusti, despite Maximian’s having been in the position for only nineteen years. During their meetings, Diocletian either convinced or ordered Maximian to step down from the Augusti and enter civilian life.
On May 1, 305, Constantius and Maximian were back in Milan, while Diocletian had joined Galerius at Nicomedia. As planned, the two Caesars stepped up to the rank of Augusti, with Galerius taking over the east and Constantius the west. Diocletian, likely at the behest of Galerius, had already selected the two new Caesars. One was Flavius Valerius Severus, a close military ally of Galerius, and the other was Galerius’s nephew, Maximinus Daia. Diocletian was clearly trying to demonstrate that the rank of Caesar was going to be chosen by merit and not birth. But in doing so he infuriated both Constantius and Maximian, who both expected to have their sons declared Caesars and then Augusti.
Consequently, all of Diocletian’s work was already unraveling as he made his way to his retirement palace in Split (in modern Croatia). First, the Empire was split into four praetorian prefectures, each consisting of three provinces and each controlled by one of the tetrarchs. This had not been deemed necessary while Diocletian ruled, since no one disputed that he was the senior Augustus and had ultimate control of the entire Empire. In the new arrangement, Galerius ruled the three Balkan dioceses, Maximinus ruled from the Dardanelles to Egypt, and Severus controlled Italy, Africa, and Spain, leaving Constantius with Gaul and Britain. Of strategic note here is that Egypt is for the first time officially recognized as being controlled by the east. Ever since Augustus’s reign, Egypt, because of its large grain surplus, had been recognized as a province of special concern to Rome, which always had first call on its grain surplus. While Rome could still bid for Egypt’s grain, this change in control is a clear acknowledgment that other cities, particularly in the east, now rivaled Rome’s claim as the leading city of the Empire.
Soon after these political arrangements were made, Constantius demanded that Galerius send his son, Constantine, who had been working with Galerius in the Balkans, to join him in the west.13 Before the end of 305, Constantine had joined his father on campaign in northern Britain. When Constantius died in 306, the army, which had a much higher regard for hereditary rights than did Diocletian, declared Constantine emperor. Galerius, who viewed himself as the leading Augustus after Diocletian’s retirement, had to acquiesce to Constantine’s elevation, since Maxentius, supported by his father, who came out of retirement, also declared himself Augustus. According to Zosimus, “Maxentius . . . could not endure the sight of Constantine’s good fortune, who was the son of a harlot, while himself, who was the son of so great an emperor, remained at home in indolence, and his father’s empire was enjoyed by others.”14 When Severus attempted to crush Maxentius’s revolt, he found that his army remained loyal to Maximian, who had commanded them for two decades. He was placed under house arrest and later murdered. With Severus removed, Galerius was forced to accept the rise of both Maxentius and Constantine, but he only did so as Caesars.15 Constantine returned to Gaul, where, after leading a campaign across the Rhine to confirm his status as a military commander, he enhanced his position further by marrying Maximian’s daughter, Flavia Maxima Fausta, thereby making his co-Caesar, Maxentius, his brother-in-law.
Despite these arrangements, war was looming when Diocletian was asked to come out of retirement and settle affairs. On November 11, 308, with both Diocletian and Maximian present, Galerius proclaimed one of his allies, Valerius Licinianus Licinius, as Augustus in place of Severus; Constantine was recognized as Caesar and, along with Maximinus, assumed the title “Son of the Augusti.”16 Diocletian then returned to his palace at Split, where he spent the remainder of his life growing cabbages. For the tetrarchy, however, there was no such peace. Like all of Rome’s emperors, they had a strategic imperative to prove they were worthy to rule, and that could still only be accomplished in battle.
Constantine, as early as 307, crushed a large band of Franks across the Lower Rhine and afterward had their kings fed to the beasts in the amphitheater at Trier. In 308 he launched a punishing invasion of his own across the Rhine. During that same year Galerius was engaged in his own reputational mending by slaughtering the Carpi and their allies along the northern side of the Danube. Licinius would take over these duties in the following year. In 308, and throughout the following year, Maxentius was battling in Africa against the governor of Numidia, L. Domitius Alexander, who had raised the standard of revolt.17 For as long as Galerius lived, these political arrangements held, just barely. But in 311 Galerius died, as did Diocletian, ending the brief period of stability.
In 310 Maximian, taking advantage of Constantine’s being once again called away to battle the Franks, revolted, telling the troops that Constantine had died in battle. But when Constantine appeared before the walls of Marseilles, where Maximian was holed up, the defenders surrendered and Maximian committed suicide, on the strong suggestion of Constantine. The original four members of the tetrarchy were now dead. In their place stood four new emperors who held little trust for one another. In the east, Licinius and Maximinus Daia were already locked in a power struggle, while Constantine and Maxentius kept a wary eye on one another in the west. In 312 Constantine aligned himself with Licinius against Maximinus and Maxentius. He could hardly have done otherwise: if he left Licinius unsupported he would be crushed between the other two armies, leaving him alone to fight the combined power of both.
In 313 Maximinus moved out of Antioch with seventy thousand men. Licinius, caught off guard by the speed of his rival’s preparations and advance, was still in Milan when Maximinus arrived opposite Byzantium. The reason for Maximinus’s speed was soon discovered, since, owing to faulty preparations, his army had suffered considerable attrition during its rapid march, with a concomitant fall in the army’s morale. A fast-reacting Licinius was able to gather strength as he marched across the Balkans, gathering forces as he advanced. When the armies met at Tzirallum (modern Corlu), Licinius was still outnumbered by more than two to one. But, as in many battles of the ancient world, discipline and experience decided the day. According to Lactantius, the battle was a one-sided slaughter:
So the two armies drew nigh; the trumpets gave the signal; the military ensigns advanced; the troops of Licinius charged. But the enemies, panic-struck, could neither draw their swords nor yet throw their javelins. . . . Then were the troops of Daia [Maximinus] slaughtered, none making resistance; and such numerous legions, and forces so mighty, were mowed down by an inferior enemy.18
Maximinus retreated toward Syria but was relentlessly pursued by Licinius, and, once besieged in Tarsus, committed suicide. Licinius, without opposition, then assumed sole control of the Roman east.
Even before all of this took place, Constantine in 312 struck like a thunderbolt in the west. With an army reportedly numbering ninety thousand infantry and eight thousand cavalry, much of it raised from barbarian tribes, Constantine crossed the Alps.19 Gibbon reminds us that at least half of this great army would have to be left to guard the frontiers, and at this stage of the Empire’s history, a mobile force numbering about forty thousand is close to the maximum that can be sustained for a prolonged period.20 They would face almost four times their number, maintained by the superior supply system in Italy. But as with Licinius at Tzirallum, quality counted for more than numbers. Gibbon’s florid prose captures the difference:
But the armies of Rome, placed at a secure distance from danger, were enervated by indulgence and luxury. Habituated to the baths and theatres of Rome, they took the field with reluctance, and were chiefly composed of veterans who had almost forgotten, or of new levies who had never acquired, the use of arms and the practice of war. The hardy legions of Gaul had long defended the frontiers of the empire against the barbarians of the North; and in the performance of that laborious service, their valor was exercised and their discipline confirmed. There appeared the same difference between the leaders as between the armies. Caprice or flattery had tempted Maxentius with the hopes of conquest; but these aspiring hopes soon gave way to the habits of pleasure and the consciousness of his inexperience. The intrepid mind of Constantine had been trained from his earliest youth to war, to action, and to military command.
Maxentius, through faulty intelligence, had placed a large force guarding the pass near Susa (Segusium) and was therefore taken by surprise when the Rhine legions descended from the pass at Mont Cenis. Moving rapidly, Constantine cut off and destroyed the force at Susa and took the city by storm. Speed was crucial: Constantine was only too familiar with the many invasions of Italy that had failed because they had lost momentum by engaging in long sieges. Just days after the victory at Susa, Constantine’s army was at Turin, where it defeated another of Maxentius’s separated field forces. In a battle narrative reminiscent of Aurelian’s battles against Zenobia, Constantine’s forces outmaneuvered, enveloped, and then finally defeated a large force of mailed heavy cavalry, which they beat down with spiked clubs. The people of Turin, not wishing to undergo the ravages of being sacked, closed their gates to Maxentius’s routed force but opened them to Constantine.
Constantine immediately marched east to engage a third Maxentian army in the Po Valley. This army, commanded by the praetorian prefect, Ruricius Pompeianus, advanced west to meet the invader outside of Verona, which Constantine had been forced to invest. Again, we turn to Gibbon for a description of events:
The emperor, attentive to the motions, and informed of the approach of so formidable an enemy, left a part of his legions to continue the operations of the siege, whilst, at the head of those troops on whose valor and fidelity he more particularly depended, he advanced in person to engage the general of Maxentius. The army of Gaul was drawn up in two lines, according to the usual practice of war; but their experienced leader, perceiving that the numbers of the Italians far exceeded his own, suddenly changed his disposition, and, reducing the second, extended the front of his first line to a just proportion with that of the enemy. Such evolutions, which only veteran troops can execute without confusion in a moment of danger, commonly prove decisive; but as this engagement began towards the close of the day and was contested with great obstinacy during the whole night, there was less room for the conduct of the generals than for the courage of the soldiers. The return of light displayed the victory of Constantine, and a field of carnage covered with many thousands of the vanquished Italians. Their general, Pompeianus, was found among the slain; Verona immediately surrendered at discretion, and the garrison was made prisoners of war.21
With Verona and Milan in his hands, Constantine now controlled all of northern Italy, and the road to Rome was open. It had been a remarkable military feat, comparable to Napoleon’s campaigns in the same region in 1796 and 1800.22 One must also note that in very recent Roman history, three generals of better-than-average ability and arguably possessing greater advantages than Constantine—Maximinus Thrax, Severus, and Galerius—had all failed in their recent attempts to conquer Rome. The difference is found in the speed with which Constantine conducted operations, which left his enemies in a state of dazed confusion.
Still, even after his defeats in the north, Maxentius remained a formidable foe. He had a large army protected by Rome’s Aurelian Walls and supported by recently stocked granaries. He also had the full support of the praetorians, who rightfully feared their own destruction if Constantine was victorious. The odds are very good that he could have withstood a siege, which, in the absence of any knowledge of germ science was always the great killer of invading armies, once they had settled-in to conduct a siege. But, probably because he could not trust in the support of the city’s populace, which had recently shown their contempt at the Roman Circus, followed by an attempt to storm the imperial palace, he decided to meet Constantine in open battle. He did so believing that he had the support of the gods, as he had consulted the Sibylline Oracles and took comfort in their prediction that “whoever designed any harm to the Romans should die a miserable death.”23 Of course, oracles only survive because their pronouncements are masterpieces of ambiguity. In this case, the one bringing “harm to the Romans” could just as easily be interpreted as Maxentius himself. Unknown to Maxentius, Constantine was also trusting his fate to faith; in this case, the Christian god. On the day of battle, the shields of Constantine’s army all bore the Christian Chi-Rho symbol—☧—a Christogram that stands for the name Jesus Christ. According to Lactantius, the night before the battle Constantine was commanded in a dream to “delineate the heavenly sign on the shields of his soldiers.”24 Another version of this tale, presented by Eusebius in his Life of Constantine, claims that Constantine was advancing with his army when he saw a cross of light above the sun and with it the Greek words typically translated as “By this conquer.” Later Eusebius describes the labarum, the military standard used by Constantine in his later conflict against Licinius, showing the Chi-Rho symbol.
Buoyed by the Sibylline prediction, Maxentius had a temporary bridge thrown across the Tiber River, after earlier having the sturdy permanent bridge destroyed to slow Constantine’s advance. He must have had strong faith in his ultimate triumph to have decided to fight with a river at his back against an army that had known nothing but victory against his forces. His dispositions, however, turned out to be a big mistake, as his forces, when arrayed on the far side of the Tiber with their rear lines on the banks of the river, left the army no room to maneuver or anywhere to re-form in the event of a reverse.
The Battle of the Milvian Bridge commenced at dawn on October 28, 312. In terms of its impact on the course of world events, it was more decisive even than the Battle of Actium over three centuries before. Supposedly, the battle began when Constantine personally led one of two cavalry charges upon the cavalry guarding his rival’s flanks. This attack quickly overthrew Maxentius’s horse and decided the day. Typically, victorious cavalry depart the battlefield in wild pursuit of a beaten foe. One has only to look at the training Hannibal gave to his cavalry before the Battle of Cannae, and Scipio before the Battle of Zama, to see how difficult it was to restrain this impulse. But this time the Tiber stopped the pursuit, allowing the cavalry to rapidly re-form and strike the unprotected flanks of the infantry. At that same moment, Constantine’s infantry stormed forward, routing Maxentius’s army. The single improvised bridge could not bear the weight of the retreating troops and collapsed, likely taking Maxentius with it. Thousands of other desperate troops rushed into the fast-flowing river and were swept away, while thousands more were slaughtered on the banks by an implacable foe.
Constantine was now the sole ruler of the Western Empire, as well as its first Christian emperor. In 313 Constantine met with Licinius in Milan and agreed to divide the Empire, a deal sealed by Licinius’s marriage to Constantine’s sister, Flavia Julia Constantia. This meeting is more famous, though, for the Edict of Milan, in which both emperors agreed to grant tolerance to Christianity and all religions within the Empire. Christians were also given the right to reclaim all the property they had lost during Diocletian’s persecutions. Licinius had to depart the conference early to deal with Maximinus’s attack, discussed earlier. Despite their agreement, and not for the first time, the Empire proved too small to hold the ambitions of two co-rulers.
Constantine’s relationship with Licinius rapidly broke down after Licinius refused to turn over the ringleaders of a plot on Constantine’s life who had run taken refuge in the East. The stage was once again set for war. The first battle took place at Cibalae in Pannonia. It must have been an unplanned affair: Constantine had barely 20,000 troops to meet Licinius’s 35,000. To offset his disadvantage in numbers, Constantine posted his army in a narrow defile.
As Licinius approached, Constantine ordered a charge. According to Zosimus, “This engagement was one of the most furious that was ever fought; for when each side had expended their darts, they fought for a long time with spears and javelins. Only after the action had continued from morning to night did the right wing, where Constantine commanded prevail.”25 With his left wing collapsing, Licinius judiciously ordered a retreat, but for most of his army it was too late. His cavalry appears to have ridden off to Sirmium, where Licinius collected his treasury and family before moving east to collect a new army. His infantry was left to be enveloped and slaughtered, and as many as twenty thousand of them were killed. As he retreated, Licinius elevated one of his generals, Valens, to the rank of Caesar, a move that, for unspecified reasons, enraged Constantine. One suspects that he saw himself as the senior Augustus and believed it was his right to name Caesars.
With Valens’s aid, Licinius formed another army in the vicinity of Adrianople. It is likely that because Valens was an Illyrian he was able to gather a significant number of veterans from along the Danube. Constantine, after securing Sirmium, sent a small force of about five thousand men ahead to make contact with Licinius while holding the bulk of his army in the Balkans to await developments. After securing his rear and gathering additional forces, Constantine advanced to Philippi, where he met peace envoys dispatched by Licinius. Confident that he had Licinius nearly beaten, Constantine spurned the offered peace. In January 317 the two armies met at Mardia (modern Harmanli, Bulgaria) in a conflict known as the Battle of Campus Ardiensis. The battle was a long and bloody affair, and as dusk approached the outcome remained in doubt. According to Zosimus, this is when the five thousand soldiers sent to watch Licinius made their appearance on the battlefield.26 Approaching unseen from behind a line of hills, this force fell on Licinius’s rear and turned the tide. Still, Licinius’s veterans were able to draw off during the night and retreat to the northwest. Constantine assumed that his foe was fleeing east to cross back into his bases in Syria and ordered an immediate pursuit. This time his aggressiveness cost him: as he marched east, he left Licinius’s army sitting in his rear along his lines of communication and supply. Licinius was in a good position geographically, but he was fighting with an army shaken by two defeats. So he again sent peace envoys to Constantine, who was now prepared to listen, but not until Valens was deposed.
With Valens pushed aside, a peace agreement was soon hammered out. Reflecting their respective military positions, the peace was very much in Constantine’s favor. Licinius ceded all his European territories except for Thrace to Constantine and either agreed to execute Valens or did so on his own accord to prove his submission. The two Augusti also agreed to appoint new Caesars, and Constantine’s sons, Crispus and Constantine II, along with Licinius’s young son, Licinius II, were all soon elevated to the purple. Finally, Licinius was forced to recognize Constantine as his senior and follow all of his imperial orders. The treaty gave Constantine control of the dioceses of Pannoniae and Moesiae, consisting of eighteen imperial provinces and substantial military infrastructure that included numerous military bases, minting centers, and three fortified imperial residences—Sirmium, Serdica, and Thessalonica. At the war’s end, Constantine governed eight of the Empire’s twelve dioceses, leaving Licinius with only Thracia, Asiana, Pontica, and Oriens.27
The peace lasted for six years, but it was always a tenuous arrangement, only maintained for as long as Licinius was willing to take orders from Constantine. But diverging dynastic ambitions and religious positions slowly eroded the peace. The final break was a result of Licinius’s repudiation of the Edict of Milan. In December 323 Constantine ordered a public beating or a heavy fine for any imperial officials forcing Christians to participate in pagan sacrifices. Licinius pointedly refused to comply and instead increased his persecution of Christians within his domain. Consequently, both emperors spent the winter of 324 girding for war. Even at the time, the renewed conflict was seen by participants and observers as war driven by religion, with Constantine now firmly a Christian and Licinius making a final stand for the continuation of Rome’s pagan traditions.
Constantine, in Sirmium, ordered his son Crispus to join him. Crispus, by now in his mid-twenties, had developed his own reputation as a general in campaigns against the Franks in 319 (when he was still a teenager) and the Alemanni in 323. He arrived in Sirmium with a large force of Gallic troops, all veterans of several campaigns and devoted to the house of Constantine. Constantine opted for a two-pronged campaign in which he would advance with the army into Thrace, while Crispus led a naval assault on the Hellespont that, if successful, would cut Licinius’s army off from its base in Syria.
In June 324 Constantine led his army east, and by July he was before Adrianople (modern Edirne), where he found Licinius’s larger but mostly raw army in a strongly fortified position. After several days of assembling his army and daring Licinius to advance upon him, Constantine struck on a new idea. He created a diversion by employing a large number of soldiers to cut down trees and pretend to be building a bridge across the Hebrus River, behind which much of Licinius’s army was sheltering. After the Licinian forces focused their attention on the fake crossing site, Constantine hid eight hundred of his best cavalry and five thousand infantry and archers in a thick forest at the far end of his line. The next morning, July 3, he personally led them across a fordable section of the river and fell upon the surprised flank of Licinius’s army. As the Licinian force degenerated into a chaotic mass, Constantine’s heavy infantry forded the river and drove forward. During the battle Constantine directed the special guard placed on the Labarum to move the sacred Christian talisman to any area where his soldiers seemed to be faltering. This seemed to embolden his troops and frighten the Licinian forces. Late in the day, the last resistance was broken by a cavalry charge led by Constantine in which he suffered a wound to his thigh. Licinius retreated with the remnants of his routed army, leaving as many as thirty thousand of his men dead on the battlefield.28
Licinius reformed his army at Byzantium and prepared to endure a siege. But when his fleet was annihilated by a combination of Crispus’s attacks and a wind that smashed many Licinian ships against unforgiving rocks, Byzantium became untenable. Licinius escaped across the Bosporus and began to raise a new army near Chalcedon. Constantine followed in early September, landing on the Black Sea coast at what was called the Sacred Promontory, and then marched west. Once again Licinius had managed to assemble a large force, including reinforcements brought up from Syria and even a large band of Gothic mercenaries. Upon learning of Constantine’s approach, he took up a position at Chrysopolis (modern Üsküdar, a suburb of Istanbul) and waited. It does not take long for a good general to examine an enemy force and determine how much fight it has in it, and Constantine was as good a general as ever served Rome. Looking over Licinius’s army, he must have felt nothing but disdain, as his battle plan was nothing more than to order an immediate frontal assault upon an army larger than his own. In one overpowering charge Constantine’s army broke Licinius’s line, after which commenced the massacre of what was now a mass of nearly helpless refugees. Depending on the account, the Licinian forces lost between 25,000 and 100,000 men. Licinius, with just a tattered remnant, escaped to Nicomedia. There his wife, Constantia, daughter of Constantius I, interceded with her father on his behalf. Licinius surrendered in return for his life’s being spared. Constantine sent him to live in Thessalonica but had him executed the following year. Within two years Constantine would also order his wife, Fausta, and Crispus executed, either on suspicion of an affair or as a result of a dispute over the political control of the Empire in which the two had stood together against him.29 Whatever the real reason for the executions, Constantine soon regretted his precipitate actions and allowed his children with Fausta to remain in the line of succession.
Constantine was now the sole ruler of the Roman Empire, but the end of the civil wars did not bring an end to disputes and controversies. Foremost among these were the numerous disputes between the various Christian groups. Whole forests have been consumed to produce learned works on how Constantine attempted to settle these conflicts, starting with the Council of Nicaea, which was the first council to claim jurisdiction over the entire Christian community. It also set the precedent that emperors had a right, even an obligation, to involve themselves in the church’s business and for them to take outsized interest in spiritual affairs. From this point forward the civil government and the church became increasingly intertwined, to the point that, toward the end of the Western Empire, church officials had assumed many of the duties and much of the power of the civil administration. These religious debates are far beyond the scope of this work and will only be addressed when and if they have a major strategic impact on the Empire’s fortunes.30
One of Constantine’s first great acts was to create the city of Constantinople atop and alongside Byzantium. At the time he most likely did not mean this city to be seen as a new Rome or even as a political rival to Rome. He therefore went out of his way to make sure no one could claim he intended for Constantinople to replace Rome as the seat of imperial power. For instance, because Constantinople was to be a city greater than the other imperial residences dotting the Empire, it would need its own Senate. But when Constantine enrolled the city’s new senators, he made it clear they were not the equal of Rome’s senatorial class. That would only happen decades later, when his son, Constantius II, elevated the dignity of the eastern senators. Still, he fully intended the city to be a testament to his greatness and collected art from across the Empire to adorn the city. Furthermore, little expense was spared in a building program aimed at making the city an architectural rival of Rome. But all of this was meant to demonstrate the greatness of Constantine and not to establish a new primary capital for the Empire.
No matter Constantine’s intent, the establishment of Constantinople as the Empire’s second city had long-term strategic consequences. Once its great walls were strengthened by Theodosius II, the city was as close to impregnable as was possible in the ancient world. These walls would defend the city for another thousand years until they were blasted apart by Ottoman cannons in 1453. Throughout most of that thousand-year period the Theodosian Walls stood as a bulwark, protecting the Eastern Empire from the northern invaders that collapsed the Western Empire a century after Constantine’s reign. As the city grew and prospered, it did become a direct rival of Rome and the home of a separate eastern emperor when the Empire was once again divided.
Constantine further broke with the governance model that had held through the Antonine era by continuing to reform the Empire’s administration. In most regards he was not creating anything new, but systematizing and improving upon what Diocletian had created. His one major change was to reopen many imperial jobs to the senatorial class that had been closed to them by a succession of imperial orders in the third century. In reality, what he was doing was fusing the senatorial and equestrian classes into a new imperial order, one in which birth could help one get a remunerative and powerful position within the imperial bureaucracy but talent counted for much more. The key point is that Constantine maintained much of the structure that Diocletian emplaced in terms of provinces and dioceses. He also accepted the permanence of the four great prefectures that had become more formalized during the tetrarchy. Diocletian had assigned each of the four tetrarchs a praetorian prefect, and Constantine maintained these positions. Constantine’s great change was to remove his praetorian prefects from their military commands. They were still responsible for the training and care of the soldiers, but they did not command in the field. These praetorian prefects were instead put in charge of the massive imperial bureaucracy that dominated every corner of the Empire. In the Antonine age, many regions, towns, and cities had maintained their own local governments and political traditions. But Caracalla’s granting of citizenship to most of the Empire’s free persons made it virtually impossible for this to continue. The Diocletian-Constantinian political and administrative reforms only formalized and made permanent a process begun a century before. From now on the governance of every town, city, and province became Roman in character, as an administrative sameness and continuity blanketed the Empire.
Throughout Constantine’s reign and that of his successors, the power of a ubiquitous and ever-increasing imperial bureaucracy continued to grow as the Empire’s government was systematized across every province. As with the story of the development of Christianity within the Empire, a recounting of decades of administrative history is beyond the scope of this work and will only be addressed as it impacts the Empire’s strategic calculus. For now, it is enough to say that the imperial bureaucracy provided the emperor with a myriad of new opportunities to make his will enforceable throughout the Empire. But as these new, administratively created provinces and dioceses became responsible for the enforcement of laws, the collection of taxes, and the payment of salaries to imperial officials and soldiers within their borders, they increased the potential for fragmentation in times of crisis, since a province or the dioceses contained within its borders had everything required to govern itself if cut off from the Empire.
By 328 Constantine either thought he had settled most of the religious disputes or was ready to put them on hold to focus on other imperial business. It had been two years since he had last been in Rome, and even longer since he had been in Gaul, which had been the foundation of his power during the Civil Wars. By September of that year he was in Trier, where he made his twelve-year-old son Constantine II Caesar. He would eventually promote his other sons and a nephew to the rank of Caesar and placed one in charge of each of the Empire’s great prefectures. The tetrarchy was reborn, but this time it was a Christian dynastic tetrarchy.31 He spent several months training the new Caesar in the duties inherent in imperial rule, including a short but sharp campaign against the Alemanni to ensure that his son received recognition as a warrior leader.
Constantine then moved east to begin a campaign against the Sarmatians and Goths along the Danube. He had previously, while heading west to Trier, paused to dedicate a great bridge across the Danube at Oescus and had also ordered the construction of other bridges, roads, and encampments. These new military constructions could mean only one thing, and they did not go unnoticed by the local Sarmatian and Gothic tribes. All could see that Rome was, at least temporarily, moving to the strategic offensive. But much had changed beyond the Danube. The old tribes—Sarmatians, Carpi, Dacians, and even left-behind provincials—still populated the region, but now they were under the sway of advancing Goths. Earlier, the Goths had advanced from the Sea of Azov to cover the entire north bank of the Black Sea, now known as the ripa Gothica—the Gothic Bank.32 From there a large number of them continued to push west, and the two largest groups of these migratory tribes were identified by the Romans: the Tervingi and the Greuthungi.
Whatever preparations they had been able to make for Rome’s onslaught were insufficient, and Constantine was soon driving the tribes north toward the Transylvanian Mountains. He thus reclaimed much of the Dacian territory abandoned by Aurelian, reestablishing the buffer zone that had protected the Balkans since Trajan’s conquest. As his right to rule, like every emperor’s, remained tied to the perception of his personal glory, Constantine added Gothicus Maximus to his growing list of titles. Constantine’s oldest surviving son, Constantius II, continued the campaign for another two years, while Constantine spent the last half of 329 and half of the following year inspecting the Moesian and Thracian Dioceses. Together father and son had so thoroughly damaged the burgeoning Gothic kingdoms that the Danube was at peace for the next thirty years. One can assume Constantine was helping to secure that peace during his inspections by strengthening the crucial defensive limes in the region. We do not know what he did in Dacia after the campaigns ended. Given how rapidly the Goths returned, it is likely that the Roman armies once again abandoned the province to its fate, placing their future security entirely upon the Danubian limes.
By 335 Constantine’s attention was turning toward Persia. The Peace of Narses had held throughout his reign. Undoubtedly, the foundational reason for this long peace is explained by turmoil within the Persian Empire, but by 330 a new ruler, Sapor II, who was only a child when Constantine took sole possession of the Empire, was firmly entrenched on the throne. As Constantine aged, Sapor was maturing and gathering strength. This time the rivalry had a religious tint to it: a newly Christian Empire was butting up against a Zoroastrian Persia. Throughout Constantine’s reign, Christianity had made large gains in the east, including the conversion of Iberia and Armenia, much to Sapor’s chagrin. In 330 Tiridates of Armenia died without a mature heir. The district leaders, who had a duty to pick a new king in cases where there was no clear heir of suitable age, split their decision. So began several years of chaos, which only ended when in 336 Sapor sent his army into eastern Armenia.
This may have been a preventative invasion, since the year before, in 335, Constantine had sent his son Constantius II to Antioch, and his nephew Hannibalianus into Anatolia, with orders to prepare for war. This war was certainly going to be fought over Rome’s and Persia’s traditional geopolitical concern—control of Armenia—but this time it would be overlaid with religious ideology. The Christian communities in both Empires were drawn to support the war for reasons of faith and in retaliation for Sapor II’s brutal persecution of the Christians within his sphere of control. At the same time, Sapor II began these persecutions not because of any internal requirement but solely in the context of his rivalry with Rome.33 In 337 Constantine set out for Antioch to take personal command of the Roman armies. He certainly intended to settle the Armenia situation once and for all. Less certainly, but very likely given that he accused Sapor II of stealing gifts sent to him from India to acknowledge Constantine’s rule over them, he meant to conquer all of Persia. Constantine, after so many battles and so many victories, surely saw himself as at least the military equal of Alexander. He would never have the opportunity to prove it, for he fell ill soon after leaving Nicomedia and died at an imperial postal station on May 22.