Expatriation comes about because of an individual's deliberate act. One officially must renounce native (or acquired) legal citizenship. This means visiting a US embassy or consulate abroad, answering a standard questionnaire, then signing a formal document requesting an end to US citizenship. Subsequent US State Dept. approval usually is granted as a routine matter. In such case the person already must have acquired a new citizenship, lest they become "a man without a country."
What specific acts could cause a loss of US citizenship? In addition to a formal 'paper work' renunciation, the US has a developed body of statutory and judicial case law describing various specific acts that can cause involuntary expatriation by citizens. Under US law these acts might include voluntary military service in the armed forces of a foreign nation, voting in foreign elections, and swearing allegiance to, or accepting an official office in a foreign government.
This automatic loss of citizenship can occur despite the United Nation's 1948 "Universal Declaration of Human Rights" which states that "no one shall be arbitrarily deprived of his nationality nor denied the right to change his nationality."
In Nishikawa v. Dulles, 356 US 129 (1958), the US Supreme Court held that the government must prove by clear and convincing proof that a potentially expatriating act was done voluntarily before citizenship can be lost. In Afroyim v. Rusk, 387 US 253 (1967), the Court said an American has a constitutional right to remain a citizen until voluntary relinquishment and that intent to relinquish must be proven by the government. In Vance v. Terrazas, 444 U.S. 252 (1980), the Court said such intention to relinquish citizenship may be proven by conduct or reasonable inferences. Mere long-term residence abroad does not cause loss of US citizenship, Schneider v. Rusk, 377 US 163 (1964), not even leaving the US to evade the draft in time of war. Kennedy v. Mendoza-Martinez, 372 US 144 (1963).
It's worth noting that US law clearly confirms the right of a US citizen to "dual citizenship." That is, US persons have the right to acquire a second citizenship and passport.
Look at these pages also:
* US Citizenship & Nationality. LINK
* Renunciation of US Citizenship. LINK
(This page is compliments of THE SOVEREIGN SOCIETY OFFSHORE A-LETTER