One scenario I could see is this, and it may be a bit out there, but you are asking for scenarios
As the war stretches on in the Middle East popular support for the current administration from both the civilian and military aspects is plummeting.
Troops and Citizens alike call for a withdrawl and immediate legal action against the current government administration. Impeachment. In a bid to
regain some sort of foot hold with the citizens of the US, the government plans an early withdrawl from most major zones of operation - with the
exception of a few critical zones.
It's another sunny morning in DC. Everything will change today, as some unseen force ("Al-Quaeda", Foreign enemies, Inside job?) detonates a
nuclear device in the US Capital. As the infrastructure of the US crumbles, the European Union makes a snap decision to send military aid and relief
to the destroyed capital in the hopes of maintaining some sort of government control.
The States do not take kindly to this foreign invasion, as they have felt they have suffered enough due to the dealings of foreign nations and war.
However, in a bid to establish a new American government and reign in control from the Europeans, the States feud on where to move the new capital.
Driven by the will to power, pride, and foolishness, several states used the recalled military forces in their area to force their will upon the
others.
So begins the Second American Civil War.
--------------------------------------------------------
In all seriousness, I could see some sort of publicly revealed deception by the government - something that encompasses all citizens - to be the spark
of any sort of civil war. With the tension at an all time and the economy at a low, I find it hard to believe people
wouldn't see an overthrow
or coup as advantageous.
As Jefferson warned us at the birth of our nation (Excuse me if I get it slightly wrong) "The tree of democracy must be soaked with the blood of
patriots from time to time."