Diocletian knew he needed to reform the army. The third century Roman military struggled against barbarian invasions and foreign enemies. As a result, he moved to make the military more flexible in the face of a porous border and intransigent foes. In response to these challenges, Diocletian dramatically increased the size of the army, decreased the size of the legions, compartmentalized the empire for faster response time, rebuilt defenses, and created a better supply system.
Diocletian created a tetrarchy of four emperors to administer the empire. This allowed the emperors to respond to challenges within their principalities rather than have one man responsible for the whole empire. Emperors served both as administrators and military commanders. As a result, multiple emperors could combat multiple crises at the same time. Also, an emperor near the frontier could respond quicker to a problem than someone in Rome or elsewhere.
The tetrarchy was further divided to reduce the chance of rebellion and increase response time. Diocletian expanded the number of provinces to about 120. He further subdivided these units into 12 dioceses grouped into four prefectures. On top of this, governors lost their military authority to real military commanders stationed in the prefectures. Plus, the emperor banned the political class from military command. This lessened the chance of rebellion and kept the army in the hands of professionals.
The reorganization needed manpower. The Roman army had become undermanned compared to the size of their territory. Diocletian initiated a draft and later expanded it to compel sons of veterans and soldiers to join. The emperor increased the size of the Roman army by at least 1/3. As a result, the number of legions in the field doubled. However, Diocletian reduced the size of the legions to 1,000 from the 5,500 in the time of Augustus. This allowed for quicker movement and more deployments.
The increased army needed food and supply. The Roman economy collapsed prior to Diocletian's reign, so money was in short supply. In response, Diocletian instituted a levy system throughout the empire to meet the demand. He used a census to determine the amount of food each province must provide. Additionally, Diocletian decreed that peasants must remain on their land and could not leave once registered. This tied entire families and their descendants to landlords for generations. Diocletian's decree began the feudal system which dominated the Middle Ages.
In addition to administrative reforms and increases in the military force, Diocletian bolstered frontier defenses. He reinforced, rebuilt, and rehabilitated older fortifications throughout the empire and on the frontiers. Plus, it appears the emperor built new defenses and repaired the road network.
Diocletian found an empire improving, but still in taters following the Crisis of the Third Century. He initiated military reforms designed to create a rapid reaction force, face Rome's challenges, and bring stability to the empire. Diocletian reformed the governmental system, increased the army's size, redesigned the supply system, and rebuilt its defenses. In the end, his political reforms collapsed, but the military changes helped stabilize the empire and allowed it to continue into the fifth century.