What causes a state to unify voluntarily with another state? If realists are right, voluntary union should never happen. In their view, states value their sovereignty above all else and would never give it up without a fight. Yet the United States and Switzerland are glaring exceptions to this paradigm. If liberals and constructivists are right, voluntary unions should be much more common and actually increasing in frequency. After all, classic determinants of integration such as international trade and communication are stronger than they have ever been. Yet the number of states in the world...