You can find huge shelves of books analyzing this, attributing it to every conceivable thing, from the ol’ “lead in the water pipes” to massive structural defects stretching into remote antiquity. There’s probably something to all of them, but the biggest proximate cause is probably one the Romans themselves had identified, as early as Sallust, way back in the last century BC: lack of purpose. Having defeated Carthage and salted the earth where once she stood, the Roman “security state” — the largest and most comprehensive ever assembled to that point — simply ran out of reasons to exist.
If you’ve read much about (or especially by) them, you know that the Romans were, for lack of a better term, paranoid. It was simply incompatible with the Roman psyche to have any large, organized group of non-Romans anywhere near the Empire’s borders … so the Empire kept expanding, first reducing, then Romanizing, every conceivable threat. They kept doing this long past the point of negative returns, such that the Empire collapsed under the strain of trying to hold itself together.
Stop me if this sounds familiar: A “government” whose only claim to legitimacy is naked force. A large, increasingly rapacious, increasingly class-conscious military with no obvious enemies to fight, and no ability to subdue the ones it settles for. A large, increasingly rapacious, increasingly caste-conscious bureaucracy that views the whole “Imperial” project as one big tax farming operation. Massive, ever-increasing wealth disparities that can only be very temporarily alleviated by debasing the currency, because structural reform is culturally impossible from above, and physically impossible from below. And to top it all off, a weird, apocalyptic religious cult totally destroying the few pan-imperial cultural institutions, including the military.
The history of the Third Century is insanely complex, not well understood in many respects even by field specialists, but the gist of it is clear enough: Some general somewhere decides he’d like to have a crack at emperorizing, so he gets his troops to throw a purple toga on him and proceeds to take on the incumbent. Some other general thinks that’s a pretty nifty idea, so he does the same thing, taking on both the incumbent and the pretender. Not wanting to be left out of the fun, the incumbent’s main general murders the incumbent and takes the crown for himself, fighting whoever’s left, plus whichever of his subordinate commanders decide they want a shot at the big chair. The Senate rubber-stamps one of these clowns, or some other general who decides he’d like to throw his toga into the ring, and civil war follows civil war until one of them temporarily triumphs … only for the same thing to happen again once the shine is off the new guy (usually in less than a year or two).
They called ’em the “Barracks Emperors” for a reason, y’all. And it’s actually worse than that, since lots of times the rebel generals didn’t bother going for the big prize back in Italy. Lots of them decided they’d do just fine ruling, say, Britain, or parts of Gaul, or Asia Minor or Egypt or what have you, while the other guys slugged it out for the increasingly marginalized, if not totally meaningless, “official” title. Declare yourself the Emperor of ___, issue some slapped-together coins with your picture on them, and go nuts. The “Gallic Emperors” had a run of nearly half a century like that — not bad at all for Late Antiquity.
Severian, “The Crisis of the Third Decade”, Rotten Chestnuts, 2021-03-18.
