If you were to try to read the U.S. law concerning dual citizenship (that is, simultaneously holding U.S. citizenship and citizenship in another country) you might not find much solid information. Nowhere does the law say that a person can be a dual citizen with the United States. Then again, nowhere does it say that one can't.
Historically, the U.S. government has used this vagueness as an opportunity to make people believe that, by choosing to become naturalized U.S. citizens, they must give up all other citizenships.
The oath that people take at their naturalization swearing-in ceremony would make anyone think that they were agreeing to give up all other citizenships right then and there. The new citizen promises that he or she will:
...absolutely and entirely renounce and abjure all allegiance and fidelity to any foreign prince, potentate, state, or sovereignty of whom or which I have heretofore been a subject or citizen.
However, the fact of the matter is that the United States will not actually stop someone from keeping citizenship in another country after becoming a U.S. citizen. Nor will it cancel the U.S. citizenship of someone who becomes a citizen of another country. The key is whether the other country's laws allow it.
Many people today successfully hold dual citizenship—that is, they are simultaneously U.S. citizens and citizens of another country.
Dual citizenship can be important for a number of reasons. Some people feel a huge sense of loss in giving up the passport of the country they once called home. More practically, the laws of their home country might require giving up other important rights along with their citizenship—such as rights to a pension, to receive government-paid health care if the person becomes elderly or disabled, to vote, or to own land there.
A slim majority of countries around the globe allow dual citizenship, at least in some form. Some countries allow it automatically upon a person obtaining citizenship, others allow it after an application process, and still others offer something less than full citizenship, with or without an application.
Canada and a fair number of European countries, (for example, Albania, Bulgaria, France, Greece, Hungary, Lithuania, Poland, Switzerland, and the United Kingdom) allow dual citizenship, while most Asian countries (such as China, India, Japan, and Singapore) do not.
Look for further information on the website of the embassy of the country in question, or talk to your consular representative.
Because the U.S. government does not formally sanction dual citizenship, there are no particular procedures to follow if you become a naturalized U.S. citizen but want to keep your old citizenship. No one will give you a certificate or other evidence that the U.S. government recognizes and approves your dual status. Your home country, however, might require more.
First, find out whether your home country will cancel your citizenship if you are naturalized as a U.S. citizen. If cancellation isn't automatic, find out whether you have to take special steps to keep your home citizenship.