It might be helpful to define precisely what "martial law" is, for the context of this thread. I suspect it varies a bit, depending upon the nation and the era....
My understanding is the military has to step in because we have a situation that overwhelms the limits of the civil government.
Regardless of the reason, something has happened to send the public into a panic. The military has to spet in until our collective anxiety attack has subsided. Then we return to business as usual.
Jeanette Isabelle