How, and why, do a society disintegrate?

Question

What are the main causes?

Answer 1

it can probably almost always be traced to when people are out for the good of themselves over the good of all.

Answer 2

simply put - because of conflict.

more complicated - because of drastic conflict where the sides are ever more polarized and the "war cry"  of each side is so deeply imbedded in their make-up that to change sides would mean denying the very truth they know from being ont eh side they're on now.

Consider Middle Eastern conflicts.

Consider the Class economic struggles of America - how far away are we from social disintegration?

Consider the struggles of Ireland.

Class economics, Religion, Politics. That's usually the underlying topic of conflict.