Hm, I questioned that back in `08 -- the replies I got when I brought up my concerns of the NBC-requirements and Obama's questionable origins to my superiors were not good; it became apparent that the Army-as-an-institution (same with the other services, but I'm Army because it's better ;)) had no intention of holding to its oath to the Constitution.
The treatment of that Lt Col, Rakin [IIRC], only proved this to be true.
So, when people claim it can't happen here
or that [active duty] military men won't turn on the civilian population I take it with a grain of salt.
Yes, I believe there would be a good chunk that would reject flat-out illegal orders — but, on the other hand, such orders may be terminal of a sequence of reasonable and otherwise lawful orders. (Also, a large military unit would be helpful in the response to a false-flag operation providing security and relief [and it would work best if they were oblivious to the false-flag nature].)
I believe this will only happen when the economy crashes and it becomes clear that there is no retirement waiting for me at the end of my 30 years (which is only 2.5 years away)...and once that happens, then maybe.
But just to defend the constitution? Nope. Most people who took the oath did so because it was a means to that payday...and not because of something they really believe in and can articulate. I would be pleased if it was 50/50...but I think that's stretching it a bit.