In the world of scenario planning, the fact that something is unthinkable should not prevent us from considering it. The breakup of the Soviet Union was unthinkable almost until the day it happened. At the same time, of course, not every impossible thing will happen.
Among the most unthinkable scenarios for most Americans is the unthinkable idea that the United States could become the disunited or turn into divided states. Even though this union accumulated very slowly in the first place, and against all odds — in other words it was not inevitable — the fact that the USA will not always be as united, or at least united in the way it is now, is considered, well… unthinkable.
But as Juan Enriquez notes in his amazing PopTech talk, based on his book “The Untied States of America: Polarization, Fracturing, and Our Future”, no US president has ever died under the same flag that he was born under. That is, the borders of the United States has constantly shifted even in modern times. The last state was added in 1959 (after I was born!) and more could be added still. Americans are comfortable ADDING states, but it might not take much to subtract one. The outcome of the US Civil War has biased Americans to disbelieving in subtraction, but that might change.