Basic Trope: The United States splits into two or more nations
Straight: The US has divided into the Western American Union, the New Confederacy, and the American Dominion.
The US has split into tens, if not hundreds of different nations.
The US divides into thousands of city-states.
The US has divided into two nations.
A couple of states, or a tiny region of the US has broken off.
Even though the US has divided into several nations, the US still exists as an European Union-like organization between the new nations.
The division is mentioned in the work, however...
A) It is a footnote in history (Alice studies the Second American Civil War in class, or the historical Civil War is referred to as the First American Civil War, hinting there were more conflicts than the historical one.)
B) The work is mainly set outside of the United States and the new nations that came from it.
C) The work is mainly set in a part of the US (or one of the new nations) where there is little or no fighting.
Justified: The US fell on hard times, and since the states knew that they would have an easier time if they just secede, they left the nation.
The Federal government becomes even more powerful, assuming complete control over the US, and destroying the state system.
However, their newfound differences are too big for them to be an unified nation.
The other states, though they are no longer formally part of the same country, realize the danger of the outside threat and come to the assistance of the state under attack.
Played for Laughs: The US split up due to differences because of sports rivalries, PC vs. Mac fanboyism, people disagreeing what flavor Dr. Pepper is, Coke vs. Pepsi fanboyism, or any other minor or trivial reason.
Played for Drama: Two brothers of differencing factions during the division fight in the new Civil War and are forced to kill one another, showing the horror and tragedy of American fighting American.