It almost doesn't matter who starts it off, if it isn't directly the Government, then the government will step in and become involved. -- There are some interesting possibilities: the gang presences, the police presences, military presence all three can be involved and, interestingly, on the side of the common man. (Less likely from the gangs, but possible if things become a race-war [i.e. they'll not sit idly by and allow their families to be taken out]).
The police and military are both problematic: military especially hinges on orders, so there's some question as to whether they'd side with the common man or not. It is certainly possible that they'll start upholding their orders and then at some point start refusing them because they "cross the line." (I'd put it at a 50/50 split as to how many would side with the gov vs the Citizen.) The police are similar, except that they would likely have far less hesitancy or rejection of orders, as they've been doing many things violative of the Constitution as a matter of standard operating procedure. [This is also dependent on how close the particular police dept is to the people of the city; if it's a small tight-knit city they're likely going to be loyal to the community over the government.]
The National Guard and States is another big question -- the States themselves could easily be drawn into some conflict that the feds are involved in. Being closer to the people, I would expect the Guard to be more loyal to the citizenry... the problem is that they'd likely be far less restrained in actions conducted in a foreign state... so it is conceivable that something happens in some state and multiple Guard units are federally activated and sent in to subjugate that state (the fed-gov would claim it's doing its Constitutional duties in putting down insurrection as well as co-opting the compliance/involvement of several States), this outlines a plan that the FedGov could use to weaken states that are slipping from its control as well as putting down a state that is "rebelling" (or at least can be claimed to be).
This is completely ignoring the foreign-nation aspects of a second Civil War. If things get REALLY bad some strong nation like China [or perhaps the UN itself] could step in "for peace-keeping measures"... though the "smart" thing to do would be to wait until both sides are worn down and then "join the party".
Interesting analysis. I was more curious as to what foreign powers you think may become involved in one way or another.
If a modern civil war were to happen the first thing that would have to happen is the rebelling side would have to secure control of many states. This would have to have been done long before hostility’s started.
The State national guard would have to be purged of officers and by implication men who might choose Washington over their own neighbors. All that could be done well in advanced to hostility’s perhaps in expecting a conflict of interest.
However if hostilities start, the very first and most important objective of the rebel faction would be securing control of some nukes. If that’s not possible finding allies with the weapons would be the next highest priority. Indeed finding allies is a priority in itself eyther way.