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 continues to climb, and the most favorable arena for unification, the European Union, seems to be hitting a glass ceiling. In Uniting States, Joseph Parent argues that unions are the balancing coalitions of last resort. Elites can weld separate states into a lasting union only when facing particularly serious threats. Drawing on five major historical cases of union--the United States, Switzerland, Sweden--Norway, Gran Colombia, and the European Union--Uniting States sheds new light on political polarization, state dissolution, federalism, and the possibility of uniting without fighting.