What causes states to politically unify voluntarily? I argue that unification is an extreme alliance prompted by extreme circumstances. First, it is only possible when stringent background conditions are met: states must balance against an optimally intense, indefinite, symmetrically afflicting threat. Second, there must be a crisis trigger, a demonstrable indication of security deficiency. Third, pro-union elites must have domestic political advantage in at least two of three arenas: the media, the military, and political procedures. Since the causes of union may also be the causes of disunion, the logic of the argument is run in reverse. Using process tracing, the argument is tested in five cases: the United States, Switzerland, Sweden-Norway, Gran Colombia, and Australia. The argument performs well and has implications for American primacy, the fall of the USSR, the European Union, federalism, and the size of states.
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