What causes states to politically unify voluntarily? The answers are urgent for states seeking stability and peaceful change with their neighbors. If realists are right, voluntary union should never happen. States value their sovereignty supremely and would never give it up without a fight. Yet the United States and Switzerland are glaring exceptions to this view. If liberals and constructivists are right, voluntary unions should be much more common. Classic determinants of integration, like trade and communication, are stronger than they have ever been. Yet the number of states in the world c ... More

What causes states to politically unify voluntarily? The answers are urgent for states seeking stability and peaceful change with their neighbors. If realists are right, voluntary union should never happen. States value their sovereignty supremely and would never give it up without a fight. Yet the United States and Switzerland are glaring exceptions to this view. If liberals and constructivists are right, voluntary unions should be much more common. Classic determinants of integration, like 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 unify states only when threats are optimally intense, indefinite, and symmetrical. In these conditions, pro-union elites must take advantage of a security crisis and compete successfully in two of three arenas: the media, the military, and political procedures. Five cases test the logic of the argument: the United States, Switzerland, Sweden-Norway, Gran Colombia, and the European Union. Parent challenges Machiavelli’s classical realist view, and sheds light on political polarization, state dissolution, federalism, and how to overcome anarchy peacefully.