A recent comment by my friend and colleague Davefairtex on the Roman Empire's self-destructive civil wars that precipitated the Western Empire's decline and fall made me rethink what I've learned about the Roman Empire in the past few years of reading.
Dave's comment (my paraphrase) described the amazement of neighboring nations that Rome would squander its strength on needless, inconclusive, self-inflicted civil conflicts over which political faction would gain control of the Imperial central state.
It was a sea change in Roman history. Before the age of endless political in-fighting, it was incomprehensible that Roman armies would be mustered to fight other Roman armies over Imperial politics. The waste of Roman strength, purpose, unity and resources was monumental. Not even Rome could sustain the enormous drain of civil wars and maintain widespread prosperity and enough military power to suppress military incursions by neighbors.
I now see a very obvious trajectory that I think applies to all empires that have been too successful, that is, empires which have defeated all rivals or have reached such dominance they have no real competitors.