Few powers are more fundamental to sovereignty than the control over immigration and the vesting of citizenship in aliens (naturalization). According to the Declaration of Independence, “obstructing the Laws for the Naturalization of Foreigners” was one of the grievances that led the American colonists to break with Britain.

Under the Articles of Confederation, each state retained authority over the naturalization of aliens. This resulted in widely varying state practices, which James Madison in The Federalist No. 42 called a “fault” and “defect” of the Confederation. At the Constitutional Convention, there was virtually no opposition to moving the naturalization power from the states to the new national government, and in the ratification debates only a handful of Anti-Federalists even raised the issue.

Madison seemed to speak the sentiment of most when at the Convention he expressed his wish “to invite foreigners of merit & republican principles among us. America was indebted to emigration for her settlement & prosperity.”

Congress passed the first “uniform Rule of Naturalization” under the new Constitution in March 1790. It allowed “any alien, being a free white person” and “of good character” who had resided in the United States for two years to become a “citizen of the United States” by taking an oath in court “to support the constitution of the United States.” Although Alexander Hamilton had argued in The Federalist No. 32 that the power to establish “‘an uniform rule of naturalization.’ . . . must necessarily be exclusive; because if each State had power to prescribe a distinct rule, there could not be a uniform rule,” some states continued to naturalize foreigners even after Congress had acted. In 1795, Congress claimed exclusive authority over naturalization by establishing new conditions—“and not otherwise”—for aliens “to become a citizen of the United States, or any of them.” In Chirac v. Lessee of Chirac (1817), the Supreme Court affirmed that “the power of naturalization is exclusively in congress,” notwithstanding any state laws to the contrary.

Individual naturalizations following Congress’s “uniform Rule” were not the only avenues to citizenship for those who were not American citizens by birth. The incorporation of the Louisiana Territory and Florida into the Union in the first decades of the nineteenth century raised the issue of whether the national government through treaty or law could vest citizenship collectively. A federal circuit court in 1813 and then the Supreme Court in American Insurance Co. v. 356 Bales of Cotton, Canter (1828) upheld collective naturalization. Moreover, in 1848 the Treaty of Guadalupe Hidalgo, which ended the Mexican-American War, offered the Mexican inhabitants of the territories ceded to the United States the option of maintaining their Mexican citizenship or, if they made no such request, becoming American citizens.

From the beginning, American naturalization law and practice assumed that a free citizen of one country had the right to transfer his allegiance to another if the latter allowed: hence, the provision of the 1795 law that required the new citizen to “absolutely and entirely renounce” any previous allegiance. However, this essential element of social-contract theory—that political communities are the free association of individuals to promote their mutual security and happiness—violated settled European norms. Sir William Blackstone had written in Commentaries on the Laws of England that the “natural allegiance” owed by all those born within the sovereign’s domain could not be “forfeited, cancelled, or altered” by any act of the subject himself, including moving to another country and “swearing allegiance to another.”

This conflict of views on the legitimacy of voluntary expatriation led to considerable conflict between the new nation and both Britain and France, especially when the latter two nations captured on the high seas and impressed into their naval service former nationals who had moved to the United States. This was one of the American grievances that led to the War of 1812. As late as the 1860s, the British government refused to recognize the American naturalization of former Irish subjects. In response, Congress passed the Expatriation Act of 1868, which declared that “the right of expatriation is a natural and inherent right of all people, indispensable to the enjoyment of the rights of life, liberty, and the pursuit of happiness.”

Key criteria for citizenship of the Naturalization Act of 1795 remain part of American law. These include: (1) five years of (lawful) residence within the United States; (2) a “good moral character, attached to the principles of the Constitution of the United States, and well disposed to the good order and happiness of the United States”; (3) the taking of a formal oath to support the Constitution and to renounce any foreign allegiance; and (4) the renunciation of any hereditary titles.

Current law, which is much more detailed than the first naturalization statutes, also requires competency in the English language and excludes those who advocate world communism or the violent overthrow of the government of the United States. Also, current law prohibits discrimination in naturalization on the basis of race, sex, or marital status. Federal agencies have expanded the elements of the oath to require a solemn commitment “to support and defend the Constitution and the laws of the United States against all enemies, foreign and domestic; . . . to bear true faith and allegiance to the same; and . . . to bear arms on behalf of the United States when required by the law, or . . . to perform non-combatant service in the Armed Forces of the United States when required by the law” (with exceptions for conscientious objectors).

Federal law and regulations establish procedures, administered by the U.S. Department of State, by which Americans can voluntarily renounce their citizenship. In addition, federal law lists a variety of acts that shall result in the loss of citizenship if “voluntarily perform[ed] . . . with the intention of relinquishing United States nationality.” These include obtaining naturalization in a foreign state; declaring allegiance to a foreign state; serving in the armed forces of a foreign state as an officer or when the foreign state is engaged in hostilities against the United States; and, in some cases, serving in governmental office in a foreign state.

Prior to several important Supreme Court decisions in the last half of the twentieth century, federal law had also required loss of citizenship for, among other acts, voting in a foreign election; deserting during wartime; leaving the country during wartime to evade military service; and, for those who acquired dual nationality at birth, voluntarily seeking or claiming the benefits of foreign nationality and residing in the foreign state for three years continuously after the age of twenty-two.

Although the Supreme Court in MacKenzie v. Hare (1915) upheld Congress’s power to expatriate, in 1958 the Court began to cut back on Congress’s power in a number of closely decided cases. Although it upheld expatriation (removal of citizenship) for voting in a foreign election, Perez v. Brownell (1958), it explicitly overruled that decision less than a decade later, in Afroyim v. Rusk (1967), ruling that a naturalized American citizen who relocated to Israel and voted in an election for the Israeli Knesset could not lose his citizenship as a result. It also overturned expatriations for desertion from the military during wartime, Trop v. Dulles (1958), and for service by a dual national in the Japanese army during World War II, Nishikawa v. Dulles (1958). In 1963, in Kennedy v. Mendoza-Martinez, the Court ruled that a citizen could not be expatriated for fleeing the country during wartime to evade military service. The following year, it extended the limits on expatriation to naturalized citizens who returned to their native countries and resided there for at least three years, Schneider v. Rusk (1964).

In Rogers v. Bellei (1971), however, the Court did uphold a statute that provided that a person who acquires United States citizenship by being born abroad to an American citizen shall lose that citizenship unless he resides in the United States continuously for five years between the ages of fourteen and twenty-eight. The Court did not backtrack on its earlier cases that held, in general, that Congress cannot take away citizenship granted by the Constitution under Section 1 of the Fourteenth Amendment, which provides that all persons who “are born or naturalized in the United States” are U.S. Citizens. That clause, the Court has held, does not allow Congress to take away that which Section 1 has granted. Bellei gained U.S. citizenship outside of the United States, the Court held, and hence Section 1 did not protect him. In 1978, however, Congress removed from federal law the continuous-residency requirements that had been upheld in Rogers v. Bellei.

Finally, in Vance v. Terrazas (1980), the Court clarified its decision in Afroyim by holding that it was not enough to show that an individual voluntarily committed an act that Congress determined was inconsistent with American citizenship. It was necessary also to show independently that the individual “intended to relinquish his citizenship.” Given the broad language of the more recent cases, it seems that no involuntary expatriations are lawful. The one exception, which applies only to naturalized Americans, is the denaturalization (and deportation) of those who became citizens through fraud or illegality. It has been applied most notably in recent decades to former Nazis who engaged in war crimes during World War II and later lied about their wartime activities either when they entered the United States as “displaced persons” or when they applied for citizenship.

Until recent decades, American public policy consistently prohibited dual citizenship. Most notably, since 1795, Congress has required that all candidates for naturalization formally renounce allegiance to their native land and any other foreign power. That requirement remains a part of national law and is an integral element of the citizenship oath. Also, as noted above, the Treaty of Guadalupe Hidalgo required the residents in the ceded lands to choose between Mexican and American citizenship. The rationale for such policies is that citizenship demands undivided loyalty to one country.

Yet today there are millions of American citizens who are also citizens of other countries. Many are naturalized American citizens whose native countries do not recognize the renunciation of loyalty that their native citizens make in the American citizenship oath. Others are the offspring of one American parent and one foreign parent, deriving citizenship from both sides, or foreign-born children adopted by American parents. Others are those who are natural born U.S. citizens who later become citizens of a foreign country. Because the courts now prohibit the government from expatriating those who maintain an active citizenship in a foreign nation (some American citizens have even held political office in other countries), dual citizenship has become a fact of American life.