In the face of the constant threats, and as his vision of the empire continued to evolve, Diocletian took the next step he deemed necessary to ensure the stability of the empire. He chose the men (the “caesars”) who would succeed himself and his colleague. For Maximian, who had a son, he chose Maximian’s Praetorian prefect, Constantius the Green, a Dardanian and a proven general. For himself he chose Galerius, an able, if blunt, general. The two caesars were required to divorce their wives and marry the daughters of their augusti. Thus Diocletian transformed the empire into a tetrarchy.
The tetrarchs divided the empire into four areas of operation. Diocletian Augustus held the east (in particular, Egypt, Libya, Arabia, Bithynia), his caesar Galerius had Illyricum and western Asia, Maximian Augustus held the west (in particular, Rome, Italy, Sicily, Africa, Spain), and his caesar Constantius the Green held Gaul and Britain. The caesars were to do the heavy fighting. If they won, so much the better. If they lost, their loss could be retrieved by their augustus wth his army; if they were judged incompetent, they could be cashiered; and, in dire emergency, the two augusti could cooperate and support each other against enemies within or without.
Along the borders of the empire and within, the tetrarchy reestablished imperial authority. In 293 Constantius the Green returned all of Gaul to the empire, and three years later he, in command of one army, and his Praetorian prefect, in command of another, crossed the English Channel. His Praetorian prefect slipped across in the fog, burned his boats, forced the rebels into battle, defeated and killed their leader, and pursued the survivors to London. The sack of London was prevented only when Constantius himself sailed up the Tiber with a second army.
The tetrarchy worked well. Maximian defeated a coalition of rebel Moors (in 297-298), Constantius the Green, though defeated once and forced to run for his life, nonetheless recovered and defeated the Alemanni in Gaul (in 298), Diocletian ordered Galerius to clear and resettle the Roman bank of the Danube (in 294-297) and then, in 297, he summoned Galerius to drive Narses, the king of Persia, out of Syria. When Galerius fell into an ambush and was forced to retreat, Diocletian received him with scorn, ordered him to walk on foot behind the Augustus’s chariot and then told him to go and redeem himself. Galerius forced the Persians to sue for peace and to cede Mesopotamia and Armenia.
The tetrarchy had succeeded in gaining control of the armies, securing the Roman borders, establishing a clear succession, and further protecting the person of the emperor by setting him apart from the rest of humanity-as a man whose imperial destiny had been established in heaven. But whose heaven, pagan or Christian? As Diocletian had opened imperial service to new classes of Romans, so Christians had risen to the highest levels of the army and the imperial staff. They were present when Diocletian presided over the sacrifices and the interpretation of the signs, and on one crucial occasion, when his soothsayers told him that they could not read the omens because the Christians were making the sign of the cross, Diocletian dismissed Christians, first from his personal staff, then from the imperial staff, and from the officer corps. He needed little persuading by Galerius that the well-being of the whole empire depended upon the preservation of the worship of the old gods and the elimination of Christianity from the empire. In 303 he ordered that all inhabitants of the empire prove their loyalty to Rome by performing a sacrifice in front of an official. The imperial bureaucracy was put to the test: to reach every inhabitant of the empire, force compliance, and issue a certificate of compliance (or identify and punish recalcitrance). In the east churches were destroyed and scripture burned, but in the west Constantius refused to authorize any persecution in his domain.
In 305, after an illness, Diocletian decided that the time had come for himself and Maximian to abdicate: Galerius became Augustus in the east, Constantius the Green became Augustus of the west, and Galerius appointed his adopted son, Maximin Daia, as his own caesar and his relative Severus as the caesar of Constantius. The succession was fraught with tension. Maximian had not wanted to abdicate, and he had a son with imperial ambitions. Constantius had an adult son of his own, Constantine, who was living at the court of Galerius, almost as a hostage. He had been schooled to believe that one day he would rule; he was an imposing figure, a leader, and sympathetic to Christianity. Constantius asked Galerius to release Constantine. Galerius could hardly refuse, but Constantine suspected that Galerius might have him murdered and, the moment he was given his release, he fled from Constantinople, galloping from way station to way station, commandeering fresh horses, and leaving the others dead. He reached his father in June 306.
Constantius introduced his son to his army and Constantine, as his father had done already, won their loyalty. When Constantius died (July 306), Constantine was acclaimed emperor by the army. He sent his portrait to Galerius with the message to either send him the purple or burn the portrait. Galerius found a brilliant compromise-he promoted Constantius’s caesar, Severus, to augustus and appointed Constantine Severus’s caesar-but the compromise enraged Maxentius, the son of Maximian, and he bribed the Praetorian Guard in Italy to hail him as Augustus. The turmoil attracted a German invasion of Gaul, but Constantine was quite capable of dealing with the Germans-Constantine’s coins show him dragging a German by the hair, kicking Germans, trampling Germans-and after he had defeated them, he enlisted a large number of Germans in his bodyguard and in the legions, both as common soldiers and as officers, thus romanizing the Germans and germanizing the Romans.
The tetrarchy was in disarray, a council summoned by Diocletian failed, six men were calling themselves augustus, and the augusti formed alliances with each other, betrayed each other, and fought each other; by October of 312 Constantine was on the march against his last rival in the west, Maxentius in Rome; and Constantine’s ally in the east, Licinius, was about to open his successful campaign against his last remaining rival in the east. On 27-28 October 312, the eve of the expected battle with Maxentius, Constantine had a vision of the Chi-Rho and heard a voice declare, “In this sign you will conquer.” Constantine had the symbol painted on his soldiers’ shields and in the morning, at the battle of the Milvian Bridge, Constantine routed Maxentius. (Maxentius was trampled to death in the rout.) Constantine needed no further proof; when, on the next day, he entered Rome, he entered Rome as the first Christian emperor. Constantine’s ally in the east, Licinius, issued a prayer (which came to him in a vision): “Supreme God, hear our prayers: we stretch our arms to You; listen, holy and supreme God.” He defeated his rival who had vowed to Jupiter that he would extirpate the Christian faith if he won.
For ten years Constantine and Licinius ruled the empire in uneasy cooperation, sometimes testing each other’s strength, sometimes cooperating against the Goths along the Danube. In 324 the cooperation broke down, Constantine proclaimed a crusade to liberate Christians from the persecution of Licinius, “a hideous serpent uncoiling,” and the two emperors fought each other for the whole of the empire at the battle of Adrianople in 324. Constantine had 130,000 infantry; Licinius had 165,000. Constantine forced Licinius to retreat into Asia Minor and in September 324 at the battle of Chrysopolis Constantine defeated him again. Licinius surrendered and was executed several months later.
Constantine was sole emperor of a reunited Roman Empire. He was forced by his experience of the recent past to react to two facts, first, that a divided army divides its loyalty and, second, that a static defense could not stop barbarian attacks. He formalized the arrangement of his predecessors, a large central reserve under his direct command and a system of forts along the limes, built to survive an attack, to stop minor raids, to impede the advance of an invading army, and to serve as rallying points and as supply depots for the central reserve. The weakness of this system is more apparent under Constantine than under the tetrarchy because the tetrarchs controlled four central armies and Constantine only one. Consequently Constantine had to increase the reaction time of the central army. He reduced the legion to about 1,000 men, incorporated mailed cavalry (the cataphracts), disbanded the Praetorian Guard (made obsolete by the new army), and replaced the Guard with small bands (500 men) of German mercenary cavalry.
The real weakness of the empire, the deteriorating economic base and the inability to find the way to economic recovery, was a result, to be sure, of the turmoil of the last century, but it was also the result of a new point of view in the empire. Diocletian had attempted to give the emperor total control of the empire and everyone in it (and thereby made the empire totally dependent upon the ability of the emperor). To meet the immediate problem of the empire Diocletian could hardly have devised a better system; on the other hand, to devise a system to ensure future economic resources and a source of manpower and the ability to meet new, unexpected challenges, he could hardly have done worse. The army had grown in size, and it and the imperial bureaucracy consumed more of the empire’s resources than the empire could afford.
A comparison of the empire of the early fourth century A. D. with the republic of the second century B. C. provides a startling contrast. The republic, faced with a number of threats, appointed republican magistrates to command the armies reacting to the threats. The republic in an emergency could field upwards of a quarter of a million men and more than twenty armies without those men or their commanders (if they were competent) being a threat to the republic. The republic could afford to field such numbers because it did not need to support the men after the campaign was over. The empire had a larger population base to draw upon but could not draw upon it because the emperors feared (rightly) that trained soldiers, returned to their homes, would be a potential army for an usurper. When the empire recruited a soldier, it was financially committed to that soldier for a minimum of twenty years. Constantine’s temporary solution was to hire cheap barbarian units instead of recruiting expensive citizen units.