This Article examines international law limitations on the ascription of citizenship and national self-definition. The United States is exceptionally generous in its extension of citizenship. Alone among the major developed states, it extends citizenship to almost all persons in its territory at the moment of birth. This birthright citizenship is constitutionally protected under the Fourteenth Amendment. At the same time that it is generous at the front end, U.S. citizenship is sticky at the back. Termination of citizenship on the individual’s part can involve substantial fees. Expatriation is contingent on tax compliance and, in some cases, will implicate the recognition of capital gains.