After World War III the United States of America as it had existed for over 200 years divided into three separate countries. The exact reasons and process by which this partition happened are ambiguous, but when the dust settled, three successor states to the USA emerged: the Russo-American Alliance, American Empire, and the rump state of the old "United States of America" proper. The American Empire is composed primarily of the southern states which used to form the Confederate States of America as well as the southern Great Plains. The American Empire retained the allegiance of most of the former United States military and major corporations, and as a result is the most powerful successor state. The industrialized regions of the northeast and west coast split away and entered into some form of alliance with Russia to deter the American Empire.
The only states that remained in the old government of the "United States of America" were a pitiful handful of landlocked states in the Rocky Mountains and northern Great Plains. Possessing little of the former USA's industrial base or population, its not clear how the remnant of the USA even managed to remain independent of the Russo-American Alliance and American Empire. It is possible that the USA played off the two other successor states against each other, convincing them that it was better to allow the remnant USA states to remain as a buffer state between the two great rivals.