U.S. citizens (or nationals) can never be stripped of their U.S. citizenship (or nationality), with limited exceptions. Also, they can give citizenship up voluntarily. This article will look at both possibilities.
Naturalized citizens who acquired their citizenship illegally (were not really eligible for naturalization) or by deliberate deceit (lied or hid important information about themselves) can have their naturalization revoked. (See 8 U.S.C. § 1451(e).)
For example, naturalized citizens who are later found to have been engaged (at any time before naturalization) in persecution or genocide, or to have been involved (at any time before naturalization, or within five years after naturalization) with the Nazi party, the Communist party, or a terrorist organization, may be denaturalized.
In such cases, denaturalization occurs by a lawsuit or criminal proceeding in a U.S. federal court.
Someone who has been denaturalized is legally considered to never have been a U.S. citizen at all. The result is that any family members who immigrated through a petition filed by that person may be placed in deportation (removal) proceedings, and sent back to their home country unless they have some separate basis upon which to claim a right to remain in the United States.
(Note that U.S. passports—a form of documentary evidence of U.S. citizenship—can be revoked for a variety of reasons, even when there is no loss of nationality involved. A serious tax debt is one example.)
As for giving up one's native-born or naturalized U.S. citizenship voluntarily, to do so, a person must both:
(See 8 U.S.C. § 1481.)
For the U.S. government to claim and prove that someone has given up U.S. citizenship would not be easy. It's not enough for the U.S. government to say that the person performed an expatriating act, such as joining a foreign military force. The person must also be shown to have intended to thereby give up U.S. citizenship.
Committing any of the following acts will create a presumption that it was performed voluntarily with the idea of giving up U.S. citizenship, although the person might later be able to rebut (disprove) this presumption:
The first five acts become effective only after the person who commits them has left the United States. The procedures for the sixth act (renunciation in the U.S.) have not yet been defined.
The act must be accompanied by a specific intent to relinquish U.S. nationality.
For all the acts listed above, it is not enough to appear to commit the act—even voluntarily—to lose U.S. nationality; the person must also commit the act in order to relinquish the nationality.
In the first two cases (naturalization in, or declared allegiance to, another country) and, to some extent, in the third case (when one's position in a foreign government does not involve policy-making), the U.S. government will presume that there is no intent to relinquish U.S. nationality. To relinquish nationality in such cases, the person will need to make a sworn statement of intent (an affidavit) to a U.S. diplomatic or consular officer.
Even in the fourth and seventh cases (joining a foreign military or being convicted of treason), the loss of nationality will not be automatic. Instead, it will depend on the U.S. Department of State's finding based on evidence that a person intended to lose his or her nationality by committing the relevant act.
Such a finding would later be open to contestation in court. (The State Department does not have a clear appeal process, but could certainly review or reconsider its decision if faced with new evidence or new legal interpretations.)
The fifth act (formal renunciation abroad before a U.S. diplomat or consular officer) provides the most straightforward and effective way of relinquishing U.S. nationality, because it allows for a quicker and more reliable determination of intent. Yet even this process does not guarantee success: Any evidence of lack of intent, or evidence of intent to retain any of the rights or privileges of citizenship, could be used by the State Department to reject the act of renunciation.
Thus, for example, an expressed desire to continue living in the U.S. would be evidence of intent to retain one's U.S. nationality, and the age, mental capabilities, demeanor or other special circumstances of a would-be renunciant could be used as evidence of lack of intent to relinquish the nationality.