Professionally i often deal with the stuff from the Ancient Roman collapse. The thing that I often go back thinking about is that it was relatively peaceful in our region, since the romanized Germanic tribes in the West and the "barbaric" ones in the East were sort of used to each other and often related. It was much more bloody in the Eastern regions, especially if you look at the Battle of Adrianople
from which the Romans never recovered (but numerous attempts were made). Why do I often think about this? Because it was actually the Barbarians who wanted to negotiate while the Romans refused although it was all written on the wall for centuries. So now in our time our Nazis, often enough from Eastern Germany of all places, compare Muslim refugees to the Germanic Barbarians from the East.
During the Roman empire's history it recruited more and more soldiers from the recently occupied territories and then send them far away to another province, so they weren't likely to take part in a rebellion (it's not that different from what the UN or NATO do, actually, and I mean that in a neutral way). And for a very long time, that system worked, until the empire became too big to manage.