Barbarians at the gate usually don't bring down once-successful civilizations. Nor does climate change. Even mass epidemics like the plague that decimated sixth-century Byzantium do not necessarily destroy a culture. Far more dangerous are institutionalized corruption, a lack of transparency and creeping neglect of existing laws.
The frightening thing about disgraced IRS bureaucrat Lois Lerner's knowledge of selective audits of groups on the basis of their politics is not just that she seemed to ignore it, but that she seemingly assumed no one would find out, or perhaps even mind. And she may well have been right.