I would agree with Mal's list but would add to it.

  • Large-scale nuclear attack (i.e. multiple cities destroyed, a la Jericho)
  • Foreign invasion (goes without saying)
  • Collapse of the dollar/economy (if the gubmint cheeze stops flowing, we'll have problems)
  • Large scale agitation by the left (if they are able to coordinate significantly and able to seize on a significant political event as an ignition point [e.g. Obama losing or racial rioting])

With the economic collapse and leftist agitation scenarios, I see those being limited to mostly urban areas. I think in those scenarios, any "acting out" by those groups in rural/semi-rural areas would be dealt with quickly enough. Not to mention their already low concentration in those areas to begin with.