By the time of Valerian’s rise in 253 c.e., Germanic tribes overran the Rhine and Danube frontiers. The Sassanid Empire of Persia would take advantage of this situation by overrunning the Romans in Syria and retaking Antioch.
Valerian moved against the Persians and was able to retake Antioch, but in a terrible Roman defeat, Valerian’s army was surrounded at Edessa in 259. The emperor was led into Persian captivity.
Such political instability allowed a man like Diocles the opportunity to rise above the station of his birth. At a time when military powers were struggling for control of the fragile Roman Empire, the status of a soldier was enhanced. After 260, Gallenius had stripped senators of their right to command legions and the sons of senators of their rank of deputy tribune.
These positions were given to career soldiers, thus opening opportunities for advancement in the rank and file. Diocles was either a freed slave or the son of a freed slave, and his education was extremely limited. Diocles joined the imperial army before 270 and in less than two decades rose far.
In the 270s mention was made of Diocles being commander of a sizable military unit on the lower Danube, roughly modern-day Bulgaria. Descriptions vary, but they agree that Diocles lacked natural heroic bravado.
Rather, he set himself to military tasks with cautious and cool precision, showing himself to be a better leader than soldier. In 282 the legions of the upper Danube declared the Praetorian prefect Carus emperor, and before the emperor Probus could respond, his own soldiery killed Probus.
Diocles’ role in the coup is unclear, but under Carus’s short reign he rose to the highest levels of military leadership. Carus elevated Diocles to command of the protectores domestici, the elite force that accompanied the emperor into battle. This gave Diocles intimate access to Carus, and he was made a consul in 283.
Following Carus’s sudden death, his son, Numerian, was named emperor, and Numerian’s father-in-law and the prefect, Aper, began gathering military power. In 284 Numerian died, and Aper, who had charge over him, was arrested by soldiers on suspicion of plotting against the emperor.
The collected army leadership halted near Nicomedia, and in a ceremony the representatives of the military units elected Diocles emperor. Diocles’ first act was the execution of Aper for Numerian’s murder. He then took the name Gaius Aurelius Valerius Diocletianus.
Diocletian still needed to subdue Numerian’s brother, Carinus, who controlled the western provinces of Rome. Constantius, the governor of Dalmatia, supported Diocletian, and Carinus was weakened by the revolt of one of his military leaders, Sabinus Julianus.
When the battle between Carinus and Diocletian took place near Belgrade in 285, Diocletian was nearly defeated when his armies’ lines were broken. But Carinus’s forces did not take advantage of their battle gains and soon discovered that Carinus was dead. Tired of battle, the soldiers of Carinus’s armies swore allegiance to Diocletian, who became sole ruler—Augustus of Rome.
In order to overcome simultaneous military emergencies continuing to befall the Roman Empire, Diocletian adopted the younger general Maximian as heir and elevated him to the status of Caesar, a powerful and historic position inferior to the status of the Augustus.
While Maximian was ambitious and able, he was a soldier of little political imagination, making him the least likely of Diocletian’s supporters to attempt to seize power. Maximian, now legal son of Diocletian and a Caesar with armies, was charged with restoring Roman authority in Gaul and the West.
A revolt in Britain soon necessitated a more drastic measure. When Carausius, a general in Britain and Gaul, was declared Augustus by his armies and challenged the mere Caesar, Maximian, Diocletian boldly elevated his adopted son Maximian to the status of Augustus. Diocletian and Maximian, both soldiers commanding armies, held real power in their dual rule.
Under their leadership the question of dividing the empire into opposed states never arose, and this reform in government would play a central role in Rome’s recovery. The two emperors could now face their enemies simultaneously in the north and east and take advantage of the fact that the fronts were composed of small, independent armies.
Diocletian sought to establish his capital in Nicomedia. After the first five years of the dual rule, Diocletian set about to further cement the government and made reforms that would consolidate advances made to secure Rome after its long decline.
Diocletian first moved to establish the Tetrarchy in order to secure succession to the throne and maintain orderly dual rule. In the Tetrarchy each Augustus would adopt into his family a Caesar as junior partner. After a decade the two Augusti would retire in favor of the Caesars, who would in turn each adopt a Caesar.
Beyond the military and governance reforms of the establishment of the Tetrarchy, Diocletian reformed the military structure, establishing frontier forces that provided defense, with a mobile reserve force maintained more centrally.
When hot spots flared, the reserves could reinforce the frontier armies. The power of military leadership was divided in an attempt to reduce their threat to imperial authority.
Diocletian’s rebuilding projects and investments in infrastructure can be seen as part of a larger plan for economic reform. Availability of goods, such as food and materials, was improved when they could more easily be transported across the empire.
However, rampant inflation was hard to control, and Diocletian engaged in an attempt at price fixing. His Edict on Maximum Prices, while ultimately failing to control prices and inflation, was a serious attempt at controlling runaway inflation, with some offenses punishable by death.
Perhaps under the influence of Galerius, who was known to be opposed to Christianity, Diocletian’s rule continued Rome’s persecutions of the Christian religion. Begun in 303, this wave of persecution would cease only in 312. A series of edicts commanded that churches and scriptures be destroyed and that church leaders be imprisoned.
Many of Diocletian’s reforms, such as the Tetrarchy, did not long survive his retirement in 305. Upon his retirement, he became the first living emperor to leave office of his own accord. The Tetrarchy did manage to halt Rome’s slide into anarchy, and the rule of the Tetrarchs renewed Roman frontier defenses.