See 51
Okay.
In regard to the questions "Who will be the first person you will shoot in this civil war? Under whose orders will you shoot?":
There is insufficient information to definitively answer those questions at this point, no matter what side of "this civil war" you would be on.
In regard to the question "Under whose authority?" (and this kind of relates to the second question above) :
Ultimately, your own authority. I read once that the army doesn't make you kill people, but it trains you how to kill people, supplies you with weapons with which to kill people, then transports you to a location in close proximity to adversarial people who have been similarly trained and supplied and lets you decide what you're going to do.
As to who should be recognized as having authority to issue orders, you'd have to decide who is trying to "defend the Constitution against enemies foreign and domestic". Either, both or any side in "this civil war" might claim to do so. If things fall apart so badly that this isn't an issue, you'd have to decide which side is most likely to "secure these rights".
As I wrote above, there is insufficient information to definitively answer your questions at this point. You or I aren't going to kick things off, but if someone else does, more information will be available.