or Little Caesar and the Consuls?
Diocletian took power by winning a civil war. He knew that he had weak skills as a military commander, but was a master of manipulating other ambitious generals.
Early in his rule, he decided that the functional capital should be located at a more central place in the empire [1]. He choose a spot at the head of the Adriatic Sea, so messages could reach the Eastern and Western extremeties of the Empire in about the same time. (He likely also chose to move out of Rome to reduce the chance of assassination. He built a “civil service” aka palace staff from the ground up to be sure of his employees’ loyalty. This is one reason why he reacted so strongly against Christians when he found that they had penetrated his staff. He considered them to be a seditious political force. Religious beliefs, as such, were of no concern to him at all.)
But even as he relocated his capital, he realized that the empire was too large to be governed by one person. He also sought a way to coopt (as we would say) potentially dangerous aspirants to the throne. He had a loyal but ambitious subordinate who sought, above all, glory and prestige. This guy was a skilled military commander. Dio thought he would take a chance that he could retain the ultimate power even if he delegated this general substantial autonomy. Dio declared the other to be co-emperor and gave him honors and featured his face alongside Dio’s on the currency. In theory Dio would govern the East and the other would govern the West where military challenges were more pressing). But they consulted frequently and historians judge that Dio functioned like an elder brother, as the ultimate authority when disagreements arose.
Much later Dio decided to take this division of the empire a step further but giving each co-ruler an assistant. The assistants were groomed to take over ruling after their boss’ retirement. Now the coinage sported four faces, which confused many tradition-minded Roman citizens. This system might have worked except that one of the assistants had an ambitious (and militarily successful) son, who sought to replace his Dad or anybody else who prevented his rapid rise. Unfortunately, the television soap opera had not yet been invented, but the story lends itself to such treatment. Rather than try to rule as long as possible, Dio retired (though he exercised some influence "behind the throne"), passing power to his assistant. But shortly after Dio’s death, the system collapsed into civil war.
I’ve presented this ”story” informally, breezily, and -- most importantly -- mixing modes of explanation and understanding. Don’t most accounts -- even those constructed carefully by well-trained historians -- mix these modes? Is that unavoidable? Is the distinction, then, between explanation and understanding artificial?
It can’t be wholly artificial, as I tried to illustrate when describing Diocletian’s fiscal policies and military strategy for the Empire. Neither he nor his advisers considered the “impact multipliers” in expenditures at the periphery as opposed to the old center cities of the Empire. (See Oxford Symposium on Coinage and Monetary, Imperial Revenue, Expenditure and Monetary Policy in the Fourth Century A.D., 1980.) But neither can we recreate for ourselves the cognitive outlook of a late Roman financier. (Can we? Some historians immerse themselves for decades in the records of a period. But even if they succeed, is their derived understanding communicable to those of us who didn’t experience that immersion?)
(Sapan Parekh suggests this web site for more information.)
[1] Thanks to James Falcone of McGill University for pointing out that Diocletian only visited Rome once in his life, some 20 years into his rule.