So the deciding factor is centralization ? If you compare it between cultures it seems like once a strong central power is established a necessary step is to also concentrate all military power in as few hands as possible . Why didn ' t events like Mongol invasions shatter those centralized states ? Becasue that ' s just what happened to the Romans . Were there just that more Germans flooding into Rome than there were step barbarians flowing into China ? To me the more interesting question is why Rome could destabilize that much . Was it pressure from outside ? Was it purely internal strife ? Plagues ? Maybe someone with some knowledge about the end of Rome in the west can answer that . <| sep |> I remember this question being asked a long time ago on the show Who wants to be a millionaire ? The guy got it wrong and lost the big prize but the answer is Canberra .