Diocletian was born about A. D. 245 in Pannonia to common, peasant class parents. He found out early that his talents suited him for a military career so he joined the Roman Army at a young age and rose through the ranks swiftly. While a young soldier in Gaul, a soothsayer made a prophecy that Diocletian would become Roman Emperor after he had "slain the Boar." Though he killed many wild boars while hunting, he remained an army officer but was not made a Roman emperor no matter how many wild pigs he had hunted. He was commander of the emperor's personal bodyguard (PROTECTORES DOMESTICI) when the emperor Numerianus was murdered by his Praetorian Prefect, Arrius Aper. The troops immediately elevated Diocletian to the throne. Diocletian's first act was to try, condemn, and execute Aper with his own hand. This man's name, Aper, means "boar", so the prophecy of Diocletian’s youth was finally fulfilled.
Diocletian realized that the Roman Empire was way too vast to be ruled effectively by one man. While an emperor was at war with the Persians in the Syrian desert, Germanic barbarians might be pouring across the Rhine frontier, burning villas, pillaging towns, slaughtering the men and carrying off the women and children. By the time word of these atrocities reached the emperor, the barbarians would be long gone. Sometimes a local army commander might rebel and take the defense of the region into his own hands, as was the case with Postumus. The local citizens would feel neglected by the rightful emperor and would transfer their allegiance to the local ruler, who could and did defend them. The local emperor would usually form a breakaway or rebel empire.
Having observed the events of the past seventy-five years, Diocletian wisely decided that he would have to choose a co-emperor to help him to rule the Roman Empire. In A. D. 286, he chose Maximinus to govern the West as Augustus while Diocletian would rule in the East. In order for the government to be passed along smoothly in case of the death of one of the Augusti, each Augustus appointed a Caesar who was designated to replace him should he die or abdicate. Thus the idea of the Tetrarchy, or rule by four men, was born. In 293, Diocletian chose Galerius as his Caesar in the East and Maximianus chose Constantius I Chlorus as Caesar in the West. All four of the men who made up the Tetrarchy were tough Illyrian soldiers like himself. Part of the reason the Tetrarchy worked so well is because these professional soldiers were intensely loyal to one another and especially to Diocletian. Another part of Diocletian's plan was for the senior Augusti to abdicate, or step down after twenty years of rule and allow the Caesars to become the Augusti in their place. The new Augusti would then each choose Caesars and thus keep the Tetrarchy complete. Diocletian believed that twenty years was long enough for any man to be on the throne. At the time of his and Maximian's abdication in 305, Diocletian was in fact worn out and prone to sickness although his partner Maximianus wanted to continue to rule.
Diocletian reorganized the army in order to deal more effectively with both barbarian invasions and rebellious generals in the provinces. Gallienus had introduced a highly mobile central army unit composed mostly of mounted cavalry. Diocletian took this idea and expanded on it. In effect, Diocletian had two separate kinds of army units after reorganization. The frontier troops, called limitanei, were stationed permanently on the borders of the empire. The central mobile army, called the Comitatensis, spent most of its time in Italy and maintained a state of readiness to march at the first sign of crisis or invasion. Diocletian also greatly increased the size of both branches of the army.
Another of Diocletian's reforms was to break up the old Roman provinces into smaller ones. Several of these provinces together made up a new division if territory called a diocese. The largest division of territory was the prefecture, each of which held more than one diocese. Diocletian assigned an administrator to each province and diocese and a praetorian prefect to each prefecture. Unlike the old praetorian prefects who were military commanders of the imperial bodyguard troops, these new praetorian prefects were civilian government officials and had no military authority whatsoever. These administrative reforms made the empire much easier to govern and did a lot to curb the power of rebellious army officers. The old provinces had been too large and the provincial governors or proconsuls could amass so much power in their hands that they were a potential threat to the emperor and the stability of the empire. Furthermore, the old provincial governors controlled not only the civil government but often had a large military force under their direct command. The top military commander in each of the new provinces could not raise enough troops to seriously challenge the imperial authority. Also, Diocletian separated the military and civil authority and gave these powers and responsibilities to different individuals. The old imperial governors could raise an army and also pass laws to raise taxes to pay for his army. Thus, they had a means of financing a rebellion if they chose to revolt, a temptation many of the governors found hard to resist. This dangerous state of affairs would come to an end under Diocletian’s rearrangement of the civil government and the military chain of command.
To deal with the devastated economy of the Third Century, Diocletian again struck good gold and silver coins. He introduced a bronze coin called a follis for use as small change by the common people. Also, he issued his famous Decree of Maximum Prices, which listed the highest price that could be charged for all kinds of goods and services, from the price of pork and wheat to charges for fixing harness and milling grain. This edict decreed severe penalties for selling goods for more than the maximum price listed, but it was only partly effective. Like all laws of its kind, it only caused people to take their goods off the open market and offer them for sale secretly on the black market at a much higher price.
In A. D. 305, Diocletian stepped down as emperor and promoted his Caesar, Constantius Chlorus to Augustus in his place. The well-known story is told of this once powerful and ruthless emperor, now an old man, peacefully growing cabbages at his villa in the town of Split in what was recently the country of Yugoslavia. Diocletian compelled Maximianus to do the same, though Maximianus very much wanted to remain senior Augustus.
If Diocletian thought he had totally retired and left Roman politics for good, he was in for a surprise. In 308, he was asked to preside over a council at Carnumtum on the Danube to decide who would be the Augusti and who would be the Caesars when his squabbling successors could not work things out peacefully. The Tetrarchy that had worked so well for Diocletian and his colleagues only worked because the four men respected and were loyal to each other. The arrangement could not hold together and fell apart after Diocletian's abdication.
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