If 0bama wanted to institute martial law, or use military troops in some other way against civilians, which officers do you think would be selected to carry out his orders? Would he pick a General Petraeus type or a Wesley Clark type?
Obviously a weasel clark type and there are plenty of those but one or even a couple of general officers will not sway any large unit or part of any of the services to go along with them if the intent is unconstitutional, such as rounding up or firing on citizen dissenters who are not breaking any law.