I think the other thing is that governments rot: almost every state begins cohesive and ends decadent. The Mongols went from cohesive to decadent in less than a generation; the North African groups that ibn Khaldun lived among tended to last 2-3 generations; the western Roman Empire lasted somewhere around 1400 years.

The history of the rulers of Byzantium/Constantinople/Istanbul also illustrates this cycle. The Byzantines began dynamic and powerful, then they became self-seeking and were conquered by the fiercely cohesive Ottoman Turks (omitting the history of the Latin Kingdom of Constantinople, which went through this cycle almost as quickly as the Mongols did), then the Ottoman Turks went from fiercely cohesive to fiercely self-interested (look up the Auspicious Incident) and collapsed in turn.

Now, the collapse part of the cycle is slowed down by a world peace that will last until/unless the United States disintegrates (and it's in trouble; look at the self-serving corruption of groups as unlike each other as the Pentagon and the labor unions) -- but I discussed both the merits and the problems of this situation in another post on this subject.