From 'Dark Age America' by John Michael Greer:

"...in the last years of Rome, so many people of the collapsing empire readily accepted the rule of barbarian warlords in place of the imperial government. That government had become hopelessly dysfunctional by the time of the... https://twitter.com/sonaliranade/status/1351016483636310016

barbarian invasions, centralizing authority in distant bureaucratic centers out of touch with current realities, and imposing tax burdens on the poor so crushing that many people were forced to sell themselves into slavery or flee to depopulated regions of the countryside..."

Further, how the breakdown of social contracts between the dominant minority and inner proles, and the eroding of the security apparatus gives rise to the barbarian 'warband': "group of young men whose sole trade is violence, gathered around a charismatic leader."

The elites forge a desperate alliance with these warbands, who gradually replace the elites. The civilization then either rapidly transitions from urban to protofeudal— or the new elites attempt to sustain the structure until it finally falls to catabolic collapse—

Or, in a sufficiently complex civilization, warlords will eat away at what us left one after another until it is finally dead.