Diocletian\'s Tetrarchy & the Dynastic Principle

[Robert Vermaat]

Constantine had all the semblance of being an heir-apparent.

The semblance, but yet he wasn't considered as such. Constantine effectively was a usurper when his father died, and his claim for the throne had to be enforced by the troops. So effectively the system worked, but Constantine broke it. After that, the Tetrarchy was dead, because after Constantine's death it was the military that did not accept anyone but the sons of Constantine. In the days and weeks after his death they killed a large number of family. I doubt that this was what he would have wanted, btw...

True, the marriage didn't occur for over a decade but, AFAIK, the engagement remained in effect. How do we account for this built-in design flaw ? How could the army or anyone else not anticipate that Constantine would one day become emperor ?

Well, that's the point, it wasn't an built-in design flaw at all. Diocletian may not have foreseen the effect that this arrangement, (no doubt wrought to strengthen the ties between the men in power) could have been seen as a promise of things to come by both Constantine and the army. But it flies in the face of the system behind the Tetrarchy itself.

Could it be that maybe Diocletian did not in fact conceive of a clear-cut Tetrarchy based solely on merit ? In other words, was the idea of the Tetrarchy constantly evolving in Diocletian's mind ? What do you think ?

I doubt that Diocletian failed to believe in his own system. But I do think that he greatly underestimated some of the powers at work. I mean, any historian of the time could have told him about the power of the armed forces, and the need to get these powers to agree.
The military may have been anxious about the 'merit' system.
On the one hand this system seems to be in their favour, as 'good' men may not make 'bad' wars (the army had no reason to think well about the past 60 years).
But on the other hand they will have felt (as we see would happen) that some ambitious men might not agree about the 'merit' of a candidate, and go for the job themselves. Which is exactly what happened with Constantine. As a result, the army chose the 'stability' of a dynasty, which was something no-one could argue about.

Of course, that did stabilise the (ever-present) problem of succession (I consider it one of the main problems of the Roman Empire) for some time. But in the end the succession of mere babies again paved the way for ‘strong men’ to become the power behind the throne, and the resulting power struggle that resulted to me is one of the major causes for the fall of the West.