Gaius Aurelius Valerius Diocletianus effectively ruled the Roman Empire in the late 3rd century and the very early 4th century A.D. (or C.E., depending on the date of the work you read about him). He did so for about 20 years, though he shared power with an Augustus of the West along with two Caesars, one of the West and one of the East, through his creation of the Tetrarchy, or the Roman version of it at least--a system by which the Empire was administered by 4 individuals.
You can still see the Tetrarchs. They are huddled together, hugging each other rather uncomfortably it seems, in a stone sculpture snatched from a Byzantine palace by the Venetians and now in place in Venice at St. Mark's Basilica.
Ruling the Empire for 20 years at that time was most impressive in itself, as the 3rd century was something of a mess. One emperor followed another with amazing rapidity. Even those sanctioned by the increasingly irrelevant and no doubt bewildered Senate were many, and those declared Augustus, unsuccessfully, by one legion or another were many as well. Diocletian was a relative unknown from the province of Dalmatia who rose up through the legions. Though he shared power he was without question the senior Augustus, primus inter pares, though that phrase was even less applicable to Diocletian than to the first Augustus who coined it or had it coined for him to help lessen the appearance of regal autocracy 300 years before.
Perhaps even more impressive is the fact that Diocletian actually abdicated, retiring to his huge palace at what is now Split. That palace resembles a Roman military encampment, called a castrum, which was typically laid out in the form of a playing-card with four major streets. Perhaps he had spent so many years overseeing the creation of castra that he wasn't certain it was possible to build anything else. He lived on for 8 years after his abdication, tending his garden, which is said to have been made up mostly of cabbages.
He's remembered most by us barbarians for his effort to impose price controls and the so-called "Great Persecution" of the Christians, policies which were failures. Price controls continue to be debated and poor Diocletian's edict is generally held up as the first example of their use or misuse as the case may be. The failure of the "Great Persecution" was trumpeted by Christian apologists at the time and has been ever since as evidence Christianity was/is divinely sanctioned, but recorded deaths of martyrs are actually very few, relatively speaking. The Christians were known to have exaggerated the severity of persecution in that case and other cases. In fact, the Christians were by that time so firmly in place among the bureaucracy and the military that there could be no extirpation of the religion even then let alone during the reign of Julian the Apostate.
Diocletian was most impressive, though, in his reformation if not reconstruction of the Roman State as a much more efficient (and large) bureaucracy. He also transformed the status of the emperor, rending that position even more autocratic and sacrosanct in some ways, but less omnipotent. His reforms of the military were impressive and effective as well. He's been considered a kind of second Augustus, restorer if not creator of the Empire, and his achievement was such as to assure its continuance in one form or another for a thousand years, in the East.
Why do homage to such a man? There is a disturbing tendency among some of us to treat as heroes or as truly great those who are, at the end of the day, great autocrats, dictators, rulers, generals. Alexander, Julius Caesar, Napoleon for example. These were men who were responsible for the slaughter of hundreds of thousands, however, and the oppression of even more. It's unclear to me why they are selected for honor, but it is difficult for me to believe that the fact they are honored by us is honorable. Is Diocletian different?
Diocletian was a cruel ruler in a cruel time, a ruthless organizer and administrator who imposed order where there had been chaos through the relentless imposition of absolute policies which sometimes worked and sometimes did not, but overall worked to prevent the Empire from dissolving. It's probable the preservation of the Empire was more desirable than the alternative.
Constantine is sometimes given the credit for the preservation of the Empire, but he merely perpetuated the reforms Diocletian imposed, and didn't obtain power in similarly chaotic circumstances. It may be that he came to be thought of in this way because of his rather ambiguous acceptance of Christianity, which sufficed to make Constantine almost holy as far as the Church and its chroniclers were concerned.
But beyond that, the preservation of the Empire and its restoration in a new form, however autocratic, was a colossal achievement, and the fact that it was accomplished by someone who started so low and was raised so high is impressive in itself, to me at least. It is also representative of one of the truly odd aspects of the Empire. Although it grew out of an oligarchy, it came eventually to allow for if it did not contemplate the acquisition of real power, even ultimate power, by certain individuals who were not aristocrats, not wealthy, not privileged in any real sense. There is something I find fascinating about such a society and such a people.
Perhaps it was a society where power was more naked, that is to say that power was less influenced by other factors traditionally recognized as bringing power. Roman imperium may have been authority "pure and simple" which could be exercised in and of itself, by anyone regardless of lineage, origin or economic status. As far as power is concerned, the more honest and simple its use and creation the more respect it should be accorded.