Let's assume for a moment that the cause of the war is that the USA fractured into 50 independent states and each state is now imperialistically attempting to conquer the others.
First of all, nuclear weapons wouldn't be an option. The only reason for the 48 contiguous states to go to war would be to conquer, and nobody wants to conquer a nuclear wasteland. Plus, since quite a few states have nuclear arms and the means to launch them, a single nuclear attack would result in a nuclear war, in which no one would win.
So, with nukes out of the question and conquest in mind, the obvious victor would be Texas. It has a large population for conscription, and its huge landmass combined with well-armed civilians would make invasion extremely difficult. It has tons of oil and raw materials, as well as plenty of farmland and ranches for food production. It also has several Navy bases, giving it a big advantage over smaller and landlocked states.
However, the trump card up Texas' sleeve would be Mexico. Due to the shared border, Texas could not only easily trade with Mexico and other South American nations for everything it needs, but it could almost certainly depend upon it for military support. You see, the reason large wars break out so often on continents that share several small independent states is that everyone fears imperialistic expansion as it nears their borders. Mexico would certainly not like the idea of Texas being invaded and conquered by an imperialistic state.
So Texas would have a large army and navy, an uninvadable homeland, plenty of food, weapons and ammunition, oil, trading partners and military support from a nearby neighbour. It doesn't look good for other states.