Citizenship of the United States
A person who is considered a citizen by more than one nation has "dual citizenship". It is possible for a United States citizen to have dual citizenship; this can be achieved in various ways, such as by birth in the United States to a parent who is a citizen of a foreign country (or in certain circumstances the foreign nationality may be transmitted even by a grandparent) by birth in another country to a parent(s) who is/are a United States citizen/s, or by having parents who are citizens of different countries. Anyone who becomes a naturalized U.S. citizen is required to renounce any prior "allegiance" to other countries during the naturalization ceremony; however, this renunciation of allegiance renunciation of citizenship to those countries.
Multiple citizenship
As a result, the theory of perpetual allegiance largely fell out of favor with governments during the late 19th century. With the consensus of the time being that dual citizenship would only lead to diplomatic problems, more governments began prohibiting it, and revoking the nationality of citizens holding another nationality. By the mid-20th century, dual nationality was largely prohibited worldwide, although there were exceptions. For example, a series of U.S. Supreme Court rulings permitted Americans born with citizenship in another country to keep it without losing their U.S. citizenship.