Further to the non-citizen soldier discussion:
The Roman army was still the most ruthlessly proficient in the world, and it had to be: frontiers needed guarding. To finance it, a vastly increased bureaucracy was in place. The provinces – stretching from Hadrian’s Wall to Iraq, from the Rhine to the Atlas Mountains – were now thoroughly Romanised and demanding a say in imperial politics. A single emperor simply could not handle the workload. So in 295 Diocletian created a system of emperors and sub-emperors.
One important result of all this was that decisions were now taken in the great imperial palaces that sprang up all over the empire (Ravenna, Trier, Split, Constantinople, etc). The city of Rome was too far from the action. The Senate still met there, but was a shadow of its former self.
I find this dispersion of decision-making power to be interesting in light of the way in which the Executive branch has usurped power from the Legislative and Judicial brances in order to enact virtual legislation through its administrative “courts”.
Like other aspects of cultural decadence, this isn’t a direct cause of societal collapse, but it is a good indicator regarding which way things are headed. Especially since Western civilization is again facing great migrationary pressure from the barbarians.