How Diocletian Rebuilt The Roman Army

The Roman army had nearly torn itself apart by the time Diocletian became emperor in 284 AD. For almost fifty years, emperors had been overthrown by their own soldiers, Germanic groups had crossed the Rhine and Danube, and the Sasanian Empire had attacked Roman territory in the east. Diocletian responded by rebuilding the military system around a simple idea: Rome needed more soldiers, more commanders, and more government support spread across the empire instead of concentrated around one emperor.

He expanded the army, strengthened frontier defenses, created additional military commands, and tied those changes to a larger administrative and tax system. He also created the Tetrarchy, placing four emperors in different parts of the empire so several military crises could be handled at once. Some of the later Roman army’s best-known features were completed under Constantine rather than Diocletian, but Diocletian laid much of the foundation.

Relief of a battle scene between the Roman legion and Barbarians from Rome. Image credit: Cris Foto / Shutterstock.com During the third century AD, the Roman Empire repeatedly faced several emergencies at the same time. Its frontiers stretched from Britain to Mesopotamia, making it difficult for a single emperor and one main field army to respond quickly. Roman forces fought Germanic peoples along the Rhine and Danube while also confronting the much stronger Sasanian Empire in the east.

The political system made matters worse. During the Crisis of the Third Century, usually dated from 235 to 284 AD, more than 20 emperors and claimants came and went. Many were killed by their own troops or defeated by rivals. At one point, the empire fractured into three major political blocs. Emperor Aurelian reunited most of the Roman world in the 270s, but the military problem remained. An emperor who took a large army east to fight Persia could leave the Rhine exposed. If he stayed near the Danube, a rival general might seize power elsewhere.

Diocletian’s reforms grew out of this problem. Rather than restoring the military system of the second century exactly as it had been, he expanded trends already underway. More forces were stationed in threatened regions, new commands appeared, and emperors increasingly operated from centers closer to the frontiers instead of governing mainly from Rome.

Diocletian’s Tetrarchy marked an important step toward divided imperial rule in the Roman Empire. Via Shutterstock / kirshu Diocletian’s most dramatic answer was the Tetrarchy. He first made Maximian his co-emperor, and in 293 AD the system expanded to four rulers. Diocletian and Maximian held the senior title of Augustus, while Galerius and Constantius Chlorus served as junior emperors, or Caesares.

Each ruler had a principal region in which he operated and a court located closer to important military frontiers. Diocletian was usually based in the eastern empire, Maximian concentrated on Italy, Africa, and parts of the western frontier, Galerius operated mainly in the Balkans and east, and Constantius focused on Gaul and Britain. These were not four independent empires with permanent borders. The empire remained one political system, but military responsibility was now spread among several emperors.

The advantages became clear during the war with the Sasanian king Narseh. Galerius suffered a serious defeat in Mesopotamia in 297 AD, then returned the following year and won a major victory in Armenia. Roman troops captured Narseh’s camp and members of the Persian royal household. The peace settlement of 298 or 299 strongly favored Rome, increasing Roman influence in Armenia and Iberia and giving Rome control of territories beyond the Tigris. Meanwhile, the other tetrarchs could deal with problems elsewhere. Rome no longer had to depend on one emperor rushing from one end of the empire to the other whenever trouble appeared.

Hadrian’s Wall, built in Roman Britain. Image credit Johnnie Shannon, CC BY 2.0, via Wikimedia Commons Diocletian also increased Rome’s military strength. Exact numbers are disputed because surviving sources do not provide a reliable empire-wide total. The army of the second century may already have numbered well over 300,000 men when auxiliary troops are included, and estimates for Diocletian’s forces vary widely. What is much clearer is that he increased the number of legions and strengthened garrisons in vulnerable regions.

The legion itself was changing. Earlier imperial legions could theoretically contain around 5,000 men, but late Roman units were often considerably smaller. Under Diocletian and Constantine, the number of named legions increased while many individual units became smaller. This allowed commanders to distribute forces among more forts and threatened sectors instead of concentrating so many soldiers in a few enormous formations.

Diocletian also invested heavily in frontier defense. Forts, roads, watchtowers, and defensive installations were built or repaired along areas such as the Rhine, Danube, Africa, Syria, and Arabia. Later in the fourth century, the army is clearly divided between frontier troops, commonly called limitanei, and mobile field armies, or comitatenses. It is tempting to credit Diocletian with creating this complete system, but the evidence is not that simple. He helped develop stronger frontier forces and maintained mobile troops around the emperors, while Constantine turned these trends into a more clearly defined field-army system.

Roman Emperor Diocletian reorganized both state and military. Via Shutterstock / German Vizulis Adding soldiers would accomplish little if Rome could not command, supply, and pay them. Diocletian therefore rebuilt much of the government around the same principle he applied to military command: divide enormous responsibilities into smaller pieces.

He roughly doubled the number of provinces, making them smaller, and grouped them into larger administrative units called dioceses. He also accelerated the separation of civilian and military authority. Earlier governors often controlled both provincial government and local troops. Under the new system, civilian governors increasingly handled taxation, courts, and administration while military commanders, often called duces, took responsibility for soldiers and defenses. The change was not applied everywhere at once, but it became one of the defining features of the later Roman Empire.

All of this was expensive. Diocletian reorganized taxation so the government could assess agricultural resources and population more systematically. The system commonly called capitatio-iugatio linked assessments of land and productive capacity with the people and labor connected to it. The goal was to give the state a more predictable flow of resources for the army and bureaucracy. Military expenses were one of the main pressures behind the broader tax reforms.

Recruitment also became more tightly organized. Military service increasingly fell on hereditary military families and compulsory levies, while recruits continued to come from across the empire and beyond its borders. Diocletian’s central problem was how to produce enough soldiers to defend several frontiers at the same time.

Diocletian did not create the finished late Roman army by himself. Some important changes usually associated with the fourth-century military, especially the fully developed mobile field armies, belong more clearly to Constantine. What Diocletian did was make the system capable of surviving the pressures that had nearly destroyed it. He added manpower, multiplied centers of command, strengthened the frontiers, expanded administration, and created a tax structure capable of supporting the larger state.

The Tetrarchy collapsed after Diocletian retired in 305 AD, but its military logic survived. Later emperors continued to rule from regional capitals, share power, maintain multiple field commands, and separate civilian administration from military leadership. Diocletian’s reforms did not restore the army of Augustus or Trajan. They created the foundation for a different Roman military, one designed for an empire that expected threats to erupt on several fronts at once.