Scholars do not have good explanations for what causes states to voluntarily form political unions -- a rare but important phenomenon. The United States did so in 1789, and today’s European Union has come close. Realists expect states to tenaciously hold on to their sovereignty and are puzzled that states would ever try to unify. Liberals believe states are driven by the search for economic gains and democratic solidarity and are puzzled that political unions are so uncommon. In this provocative but carefully argued study, Parent makes the case that states create political unions only when they are imperiled by security threats. Unification is actually a type of “extreme alliance,” in which states of roughly equal power bind together for mutual protection. Parent finds only two instances of successful political unification -- the United States and Switzerland -- which he contrasts with the failed efforts of Sweden and Norway and with Simón Bolívar’s attempt to unite South America. The book succeeds in building a realist explanation of unification. But with so few cases of voluntary political union, it is not clear whether Parent’s theory is all that illuminating. Still, the book does generate a clear prediction in regard to Europe: without an intense, enduring, and widely shared threat, the core European states will fail to merge into a true union.