Many of the scenarios focus so specifically on the federal government that we tend to forget that the United States has and always has had a perfectly tenable government structure that is not federal at all. If a Tsar Bomba took DC out the result wouldn't be Mad Max, it would be 50 functioning state governments, each of which has its own police force and military. A usurper in the White House can give whatever orders he or she likes; without 50 governors' buy-ins it's only words.
Nor will the U.S. armed forces present an unstoppable force for oppression. There are 1.2 million active duty members, only some of whom would remain loyal to a takeover that will, over time, reveal itself as obviously unconstitutional. There are 21.8 million military veterans, an equal proportion of whom have combat experience as the current active membership. We all took the same oaths.
None of these scenarios is likely. Some aren't even possible. What is likely is small-scale terrorism that may look a little like Kosovo and may look a little like Germany when Baader-Meinhof was active. Large-scale convulsions, not so much - much of this is being driven by people who would stand to lose in the presence of an international economic collapse, not gain. But even if such a collapse occurs, in the overall model all of this still can devolve into state, and even local government. It's a very strong system. Almost as if it were planned that way.
All IMHO, of course.
The only sane comment on this entire embarrassing thread.