If martial law were ever declared nationwide in order to perpetuate an existing regime...
Would the military honor its oath to "protect and defend the Constitution of the United States against all enemies, foreign and domestic"?
That would depend on who the military deems the enemy of the constitution is.
What is your first guess, as to whom the military would consider an Enemy of the Constitution, in the event martial law was declared by civilian authorities, in order to perpetuate their regime?
It depends on the regime.
This thread is starting to smell like the bush, Clinton, bush coup myth that pops up on the net every time and election year rolls around.
I'm fairly certain that the presidential coup myth is far older than the net.