In the event of CWII, who do you see getting involved in the conflict.
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".
Excellent point. If we have a breakdown of order here, all that stands between us and China swooping in, is Japan. What with China making war noise with India, Japan, Vietnam, Taiwan, Korea... well, China is rankling their swords at everyone nearby, Japan could very well end up defending us.