When someone acquires nationality or citizenship in a country where they are not native born, it is called naturalization.
Most of the time, to be naturalized, you must hold legal status as a full time resident of a country. There is usually a waiting period before you can become naturalized. You must uphold and obey the new country’s laws and often pledge allegiance to that country. At times you must give up any other citizenship that is held. Sometimes you may not hold dual citizenship. Sometimes you may not lose citizenship in the second country.
Nationality is often determined by your birth or by the birth of one or more of your parents. Sometimes, nationality is determined by a combination of the two.
At times during the 20th century there have been mass naturalizations as a result of conflict, wars or genocide. Examples are the Greeks taking in Armenians in the early 1920s and mass naturalization in Argentina resulting from an economic crisis.
There were also large numbers of stateless people created during the 20th century. Some were fleeing the Spanish Civil War; some were Armenians and Jews; some were Russians fleeing communism.
Many of the stateless found themselves interned. Often they were considered undesirable and remained “illegal”. No country wanted them for naturalization.
The Nansen passport was created for stateless people. During WWII, up to 52 countries recognized the Nansen passport as valid. With one, stateless people could often remain in a country where they were denied citizenship.
The end of WWII created another large group of stateless people and mass migrations internationally. Sometimes this new group could become nationalized based on family affiliations but often stateless people remained illegal. Some countries did accept them en masse.
There are currently about 4,000 stateless people living in the United States, and in 2010, legislation proposing a path to citizenship has been introduced.