Starchild:
I'm not sure we have any examples of "anarchy eroding into uncivilized anarchy"--do you? Sometimes, as in Somalia (or Iraq?), "anarchy" arises with the collapse of a central government, but under conditions of great violence and after a long history of extreme injustice. If everything isn't immediately hunky-dory, "anarchy" takes the blame.
When anarchy is replaced, it seems to be by governments (compare the replacement of the Articles of Confederation by the Constitution). As for Iceland, simplifying from Byock, and giving it my own slant: The introduction of Christianity in the 11th century was the beginning of the end. It had two effects. (a) It gave outsiders a voice in Iceland's internal affairs. Over time, chieftains became chieftain-priests, and their selection had to be approved by Rome. (b) It increased stratification in Icelandic society. The new chieftain-priests were more powerful than chieftains formerly were; the number of chiefs was drastically reduced, from maybe 30 or more to half a dozen; and the Church itself became a very powerful entity, and large landholder. With a smaller number of more powerful chiefs, it was harder to get justice done, especially with a grievance against one of the newly powerful parties. Eventually, someone with a grievance appealed to the king of Norway for help, and the country came under Norwegian control. The shift was gradual, since the Norwegian kings for the next couple of centuries were benign. But there was never any threat, as far as I can tell, of a deterioration into "uncivilized anarchy."
