Naturalization (or naturalisation) is the legal act or process by which a non-citizen of a country may acquire citizenship or nationality of that country. It may be done automatically by a statute, i.e., without any effort on the part of the individual, or it may involve an application or a motion and approval by legal authorities. The rules of naturalization vary from country to country but typically include a promise to obey and uphold that country's laws, taking and subscribing to an oath of allegiance, and may specify other requirements such as a minimum legal residency and adequate knowledge of the national dominant language or culture. To counter multiple citizenship, most countries require that applicants for naturalization renounce any other citizenship that they currently hold, but whether this renunciation actually causes loss of original citizenship, as seen by the host country and by the original country, will depend on the laws of the countries involved.
The massive increase in population flux due to globalization and the sharp increase in the numbers of refugees following World War I created many stateless persons, people who were not citizens of any state. In some rare cases, laws for mass naturalization were passed. As naturalization laws had been designed to cater for the relatively few people who had voluntarily moved from one country to another (expatriates), many western democracies were not ready to naturalize large numbers of people. This included the massive influx of stateless people which followed massive denationalizations and the expulsion of ethnic minorities from newly created nation states in the first part of the 20th century, but they also included the mostly aristocratic Russians who had escaped the 1917 October Revolution and the war communism period, and then the Spanish refugees. As Hannah Arendt pointed out, internment camps became the "only nation" of such stateless people, since they were often considered "undesirable" and were stuck in an illegal situation, wherein their country had expelled them or deprived them of their nationality, while they had not been naturalized, thus living in a judicial no man's land.
Since World War II, the increase in international migrations created a new category of migrants, most of them economic migrants. For economic, political, humanitarian and pragmatic reasons, many states passed laws allowing a person to acquire their citizenship after birth, such as by marriage to a national – jus matrimonii – or by having ancestors who are nationals of that country, in order to reduce the scope of this category. However, in some countries this system still maintains a large part of the immigrant population in an illegal status, albeit with some massive regularizations, for example, in Spain by José Luis Zapatero's government and in Italy by Berlusconi's government.