History of Diocletian. 

Diocletian becomes emperor. In 284 Carus was the senior Augustus and his two sons Carinus and Numerian were co-Augusti. Carus went east accompanied by Numerian, with Diocletian one of his generals, with plans to regain Mesopotamia from the Sasanians. Carinus stayed behind to take care of the rest of the empire. Carus successfully captured the Sasanian capital, Ctesiphon, but was later found dead in his tent after a thunderstorm. The romantic story is he was struck by lightning, however subsequent events suggest he was murdered by the praetorian prefect Arrius Aper. Numerian inherited the war and made peace on favorable terms. However, while returning from Mesopotamia he, too, was found dead. This time Aper was accused of murder and was executed. On the spot Diocletian was chosen emperor by the army. That left Carinus still Augustus in the west. After dealing with the usurper Julian of Pannonia, Carinus went east to confront Diocletian. The story is that Carinus was a womanizer and while Diocletian appeared to be losing a decisive battle with Carinus, Carinus was assassinated by an officer whose wife he had seduced.  Diocletian inherited the empire. 

Diocletian as Emperor.  He is perhaps best known for voluntarily retiring (admittedly, after being quite ill) after a long reign of over twenty years. Much of Diocletian's impact was not because of glamorous wars, rather because of his dramatic changes of the day-to-day administration of the empire. Of course, he is famous for introducing the tetrarchy of four simultaneous rulers, but that is only the most visible (especially on coins!) of his many administrative changes that subdivided responsibilities. Diocletian dramatically increased the number of provinces by dividing old provinces into smaller regions and he separated their civil and military positions. This made it more difficult for any governor or general to command the resources to revolt. He reformed taxes by introducing an annual budget with fixed annual tax requirements on all parcels of land, according to their productivity, to be paid in kind, and on livestock and on people (a woman counted as half a man). Taxes had not been reformed for centuries, much valuable land had been untaxed, and taxes had been collected erratically with many rich people being able to evade them. His tax Edit of 297 was intended to raise more money for the state, but in a fair manner.  Also, it hoped to reduce inflation by pegging taxes to goods instead of to gold and silver coins. (Later the system was abused by raising the tax requirements to impossible levels.) He required sons to take up the professions of their fathers--a very strict way to make sure that arable land retained farmers, armies had soldiers, and guilds had members. For example, if a man married the daughter of a miller, he was required to become a miller. Freedom was curtailed.

Modern markets rely on price incentives to shift supplies and people around to necessary and productive uses, but Diocletian and subsequent rulers saw that markets did not get the necessary jobs done, so they decreed they be filled by people who knew how to do them--the sons of people who had done them. 

As usual, there were some wars with foreign adversaries. Conflicts with the Sasanians provoked a line of defensive forts on the eastern frontier; the remains of some are still visible in the eastern deserts.Diocletian's recognition that the empire was too large for one person to rule and the west and east each needed their own emperor turned out to be correct, even though Constantine was later able to unify the empire for a few years before it was again divided among his children. 

When Diocletian retired in 305 and forced Maximian to retire with him, the Caesars became the new Augusti and two new Caesars were appointed, giving hope that the tetrarchic system would continue. However, when Constantius, a new Augustus of the Second Tetrarchy in the west, died in July 25, 306, the system of orderly succession of four rulers fell apart. Nevertheless, many of Diocletian's other administrative changes remained. Later rulers doubled down on Diocletian's methods of filling jobs and financing the government. His impact was long lasting.