As California residents may know, when an individual owes allegiance to two countries simultaneously, they are considered to be a dual national. Countries have laws that speak to this issue. In the U.S., the law does not deal with dual nationality other than to acknowledge it exists. When a foreign national becomes a U.S. citizen, the U.S. does not force the individual to choose.
A U.S. citizen may become a dual national through marriage or a foreign national may go through naturalization and gains U.S. citizenship. He or she may then maintain citizenship in their birth country. A baby born outside of the U.S. to a parent who is a U.S. citizen is a citizen of the U.S. and the birth country.
An individual who achieves dual nationality by marriage or birth may retain their U.S. citizenship. There are times when a U.S. citizen may lose their right to citizenship. When an American citizen voluntarily applies for citizenship in a foreign country, they may lose their U.S. citizenship if they do it with the intention of renouncing it.
When an individual holds dual nationality, they are obliged to adhere to the laws in both countries. The laws may conflict, and the U.S. may find it difficult to assist a dual national if they are physically in the other country. When dual nationals travel, they are expected to obtain a U.S. passport to travel into or out of the U.S. Likewise, they must use the passport they received from the other country when they enter or leave that country.
Individuals who hold dual citizenship may have questions about their status in either country. An attorney may offer guidance over tax issues or military service.
Source: US Department of State , “Dual Nationality“, November 26, 2014