[some_guy_there]

It definitely will (refuse orders), especially if Trump gives an at-face unconstitutional order (say shooting the protesters at Dakota pipeline). In normal times, the Congress will impeach the President, removing him from office. But as norms erode, that might not be the case.

The officers of US Armed forces take an oath for the constitution, and not the President. Now for this special case right now, military refusing orders will be a "good" thing. But in long run, it only means that civilian institution have eroded so far that having military deciding what is constitutional might appear to be a "good thing". So the idea isn't that military personal will use force against citizens, but rather they will not, and that will erode the system regardless.

That never has been the case yet. While the President after as President has tried to stretch his authority (while Congress receded), we haven't reached a point of resistance from extra-governmental organizations. But that's the logical direction where this is headed.