I just did a bit of a rant about Diocletian, but one of my points there, fits here. I have always read, although I'm not quite sure of the numbers, that from Diocletian on, the need to keep ever-increasing armies everywhere (including the east, after the rise of the Sassanid Persians) meant increasingly crippling taxes.

Now, what does this mean? I was stunned to read recently that a wealthy man in the U.S. right after WWII could pay easily as much as 65% of his gross income in taxes. Any society with similar taxation over decades would be hamstrung in terms of any growth, surely? And when you mix obdurate taxation with increasing barbarian incursions that destroy the BASE of taxation by destroying land and property, you've got whole segments of society being driven increasingly into poverty, surely? That, along with the fact that, more and more, the army itself was given over to duties of tax-gathering, which meant the army's efficiency (supported by the taxes) was in constant decline....

I know the general outlines of this, but I'd love to have any specifics!