Naturalization is the process by which a citizen of one nation obtains citizenship in another country. In the early twenty-first century, the naturalization process was entirely within the control of the various nation-states comprising the international system. Each state determined for itself the requirements it would utilize to permit foreigners to join its citizenry. Consequently, there was considerable variation from country to country.

Under this system, each state must first determine the number of immigrants it will permit to enter its land, for each immigrant is a potential citizen. The laws establishing immigration quotas are based on a prior determination of the national interest, and that policy can change over time or with different political leaders. For example, throughout the nineteenth century the United States placed virtually no restrictions on immigration from Europe because the American leadership wanted to populate the country with people from those lands. Simultaneously...