2012- John Liberal is elected, after a fiece campaign. His platform is very left leaning, and amongst it’s planks is gun control. Specifically, the outlawing of handguns and most longarms.
President John Liberal immediatly signs an executive order. It is immediatly contested in court, but until a decision is made, the order stands.
2013- An FBI/BATF taskforce exchanges gunfire with an elderly farmer who refuses to give up his hunting rifles. The man is killed. Several weeks later, agenst approach another rural home to seize weapons and a fierce firefight erupts. The agents are driven back, and the National Guard is called out (think Waco).
National Guard troops refuse to fire first on civilians. Many guardsmen are sympathetic to the civilians. Nobody is certain exactly what happens, but elements of the Nat. Guard are arrested and charged for aiding and abetting.
Meanwhile, armed standoffs are cropping up all over the country, and National Guard units are less and less likely to respond to these situations.
President Liberal, fearing a poltical disaster, activates the US Army, which immediatly puts down several standoffs with deadly force.
Somewhere in the MidWest, however, a National Guard unit clashes with a US Army detachment.
Things rapidly go downhill, and the US essentially splits, not geographically (at first) but politically, with the extremes of right and left actually fighting it out. Eventually both sides have enough territory to be “independant”, and one side claims to be the US, while another may also claim to be the US, or may outright declare itself a new and independant nation.
It’s possible. Not likely, but possible.