[Strawman]

Originally Posted by Deckard

If it's an oil-rich state like Texas, maybe they would be allowed to go their own way - then covertly destabilized, and if that didn't work, invaded!

OK, serious answer: I reckon it would all hang on how the secession occurred. If there was also a strong unionist movement and if it turned nasty, which is quite possible - people tend to have very strong feelings about this sort of thing - then yes, I can easily see martial law being declared. These days, the word "stability" is the politician's most useful tool.

I think you got it right in the first paragraph.....