As Fred reminded us in a comment, 8 USC § 1185(b) (Immigration and Nationality Act of 1952, § 215(b)), “Travel Control of Aliens and Citizens”, demands that:

Except as otherwise provided by the President and subject to such limitations and exceptions as the President may authorize and prescribe, it shall be unlawful for any citizen of the United States to depart from or enter, or attempt to depart from or enter, the United States unless he bears a valid United States passport.

This requirement traces its roots all the way back to the 1952 INA as first enacted. However, it’s worth remembering two rather amusing facts about the law as it originally stood: it only required United States citizens to bear a “valid passport” and not a “valid United States passport”, and it only applied in time of war or national emergency:

(a) When the United States is at war or during the existence of any national emergency proclaimed by the President, or, as to aliens, whenever there exists a state of war between or among two or more states, and the President shall find that the interests of the United States require that restrictions and prohibitions in addition to those provided otherwise than by this section be imposed upon the departure of persons from and their entry into the United States, and shall make public proclamation thereof, it shall, until otherwise ordered by the President or the Congress, be unlawful …

(b) After such proclamation as is provided for in subsection (a) has been made and published and while such proclamation is in force, it shall, except as otherwise provided by the President, and subject to such limitations and exceptions as the President may authorize and prescribe, be unlawful for any citizen of the United States to depart from or enter, or attempt to depart from or enter, the United States unless he bears a valid passport.

So when did these two facts change, you might ask?

“Passport” or “United States passport”?

As seen in the original version of the law quoted above, U.S. citizens were only required to bear a “valid passport” when entering the United States. This was reflected in the regulations promulgated by the State Department under the INA on 20 October 1966, 31 FR 13546, which restated the passport requirement without any restriction on what country’s passport it should be:

§ 53.1 Passport requirement

Under section 215(b) of the Immigration and Nationality Act (8 U.S.C. 1185(b)), it is unlawful except as otherwise provided for any citizen of the United States to depart from or enter, or attempt to depart from or enter, the United States without a valid passport.

The 1972 amendments to the regulations (37 FR 11459) added the caveat that a citizen could use a consular identification card instead of a passport to enter the U.S., while the 1986 amendments (51 FR 26247) allowed the Secretary of State to waive the passport requirement in exchange for the payment of a fee. Ironically, as late as 2006, the regulations still made no mention of “United States passport”; they were only amended to require one in November that year (71 FR 68412, 68430).

However, it seems that the State Department has always interpreted “passport” to mean solely “United States passport” — or perhaps they were simply ignorant of what the law actually said — as can be seen in this 1987 State Department Board of Appellate Review decision about a U.S.-born man who had opted for Belgian citizenship at age 22 and was thereby ruled to have relinquished his U.S. citizenship. At page 300:

Although XXX apparently received a Belgian passport in 1978 without having to take any special steps, in 1984 he was required to make specific application for renewal of that passport. Such action gives rise to a not unreasonable inference that he made a conscious decision to document himself as a foreign national rather than as a United States citizen, especially when it is recalled that he visited the Embassy in 1980 and 1984 and neither time inquired about his citizenship status or applied for a passport. By his own admission he made six trips to the United States between 1980 and the end of 1985. Each time he travelled on a Belgian passport in contravention of United States law that United States citizens must use an American passport when entering the United States (from outside the western hemisphere) or departing the country. Section 215(b) of the Immigration and Nationality Act, 8 U.S.C. 1185(b).

Arguably, a citizen’s use of a foreign passport under certain limited and unusual circumstances could plausibly be explained on grounds of convenience, not as evidence of an intent to relinquish United States nationality. However, a citizen’s regular travel over a five year period to the United States on a foreign passport can hardly be explained on the grounds of mere convenience. Such conduct is so blatantly inconsistent with United States citizenship that the fairest inference to be drawn from it is that the citizen no longer considered himself to be a United States national; such conduct is fully consistent with an intent to transfer his allegiance to the state whose passport he used with such insouciance and so consistently. In the circumstances, the Department’s inference that’s belated application for a United States passport suggests a wish to recant his naturalization does not strike us as unfair.

Was it permissible for the State Department to add words to the statute that weren’t actually there? Hardly. As the U.S. Supreme Court pointed out in United States v. Laub, 385 U.S. 475 (1967), in finding that the defendant had not violated § 215(b) by travelling to Cuba even though U.S. passports had been declared “invalid for travel to Cuba”:

Section 215(b) is a criminal statute. It must therefore be narrowly construed. United States v. Wiltberger, 5 Wheat. 76, 18 U. S. 95-96 (1820) (Marshall, C.J.). Appellees urge that § 215(b) must be read as a “border control” statute, requiring only that a citizen may not “depart from or enter” the United States without “a valid passport.” On this basis, they argue, appellees did not conspire to violate the statute, since all of those who went to Cuba departed and reentered the United States bearing valid passports. Only if, as the Government urges, § 215(b) can be given a broader meaning so as to encompass specific destination control — only if it is read as requiring the traveler to bear “a passport endorsed as valid for travel to the country for which he departs or from which he returns” — would appellees be guilty of any violation …

Under § 215(b) and its predecessor statutes, Congress authorized the requirement that a citizen possess a passport for departure from and entry into the United States, [Footnote 5] and there is no doubt that with the adoption and promulgation of the “Excluding Cuba” regulation, a passport was required for departure from this country for Cuba and for entry into this country from Cuba. Departure for Cuba or entry from Cuba without a passport would be a violation of § 215(b), exposing the traveler to the criminal penalties provided in that section. But it does not follow that travel to Cuba with a passport which is not specifically validated for that country is a criminal offense. Violation of the “area restriction” — “invalidating” passports for travel in or to Cuba and requiring specific validation of passports if they are to be valid for travel to or in Cuba — is quite a different matter from violation of the requirement of § 215(b) and the regulations thereunder that a citizen bear a “valid passport” for departure from or entry into the United States.

The Court’s attitude is clear: “valid passport” means “valid passport”, not “valid [insert additional requirements here] passport”. The State Department is just lucky no one called them on their posturing before U.S. legislators finally noticed the inconsistency between what everyone assumed the law said and what it actually said. And in 1994, Senators Ted Kennedy (D-MA) and Alan Simpson (R-WY) moved a “technical amendment” to the Nationality and Naturalization Amendments of 1993 to add the requirement that the passport used by a U.S. citizen to enter the United States be a U.S. passport, leaving us with the law as it stands today.

In 2006, Ted Poe (R-TX) attempted to even further tighten § 215(b) in his Passport for All Act (HR 4120), by curtailing the President’s authority to make general exceptions and restricting such exceptions to individual cases:

(c) For Citizens—Section 215(b) of the Immigration and Nationality Act is amended—
(1) by striking `Except as otherwise provided by the President and subject to the such limitations and exceptions as the President may authorize and prescribe, it’ and inserting `It’;
(2) by striking the period at the end and inserting `, except as provided by the President—’; and
(3) by adding at the end, as amended by this subsection, the following new paragraphs:

(1) in the case of an unforeseen emergency in individual cases; or
(2) in the case of humanitarian or national interest reasons in individual cases.’.

However, his bill died in committee.

Violation of dual citizens’ right to freedom of movement

The restriction that the harsh passport control laws would only apply in wartime was removed by Section 707 of the Foreign Relations Authorization Act for Fiscal Year 1979 (Public Law 95-426; 92 Stat. 963, 992):

Travel documentation of aliens and citizens

Sec. 707. (a): Subsection (a) of section 215 of the Immigration and Nationality Act (8 U.S.C. 1185) is amended by striking out “When the United States” and all that follows through “be unlawful” and inserting in lieu thereof “Unless otherwise ordered by the President, it shall be unlawful”.

Jimmy Carter signed this bill into law on 10 October 1978 with no comment whatsoever about this sweeping expansion of border control powers.

This introduced an imbalance. In the law as it originally stood, a U.S. citizen could be required to have a passport when entering or leaving the U.S. during wartime. However, given that it was wartime, INA § 349(a)(6) (8 USC § 1481(a)(6)) provided him with the option to renounce his citizenship in prescribed form to the Attorney General even from within the bounds of the United States, and leave the country without being subject to any of the restrictions which applied to U.S. citizens, such as the requirement to carry a valid passport. Thus, if dual citizens seeking to travel were faced with intransigent authorities who refused to issue U.S. passports to them for political reasons and persisted in their erroneous interpretation of “valid passport” to mean “valid United States passport”, they could by renouncing citizenship preserve the freedom of movement which was their natural right as expressed in the International Covenant on Civil and Political Rights, Article 12(2), that “[e]veryone shall be free to leave any country, including his own”. And in peacetime, there was no requirement for a U.S. citizen to have a valid passport when entering or leaving at all.

However, after this amendment, the requirement to hold a valid passport when entering or leaving was extended to peacetime. Although the U.S. in practice does not have exit controls, the amendment thus created the legal framework for dual citizens’ emigration (and indeed the emigration of all citizens) to be restricted regardless of the absence of war or national emergency. Interestingly, since the DC District Court has ruled that the U.S. is indeed in a “state of war” for purposes of INA § 349(a)(6), it is still theoretically possible to renounce citizenship from within the U.S.; however, the immigration reform bill put forward by the “Gang of Eight” seeks to remove that possibility.

In a minority opinion in Briehl v. Dulles, 248 F.2d 561, 583 (1957), Judge David L. Bazelon of the DC Circuit Court decried these kinds of restrictions on emigration, writing:

Almost a century ago, Congress 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,’ and decreed that ‘any declaration, instruction, opinion, order, or decision of any officers of this government which denies, restricts, impairs, or questions the right of expatriation, is hereby declared inconsistent with the fundamental principles of this government.’ 15 Stat. 223-224 (1868), R.S. 1999, 8 U.S.C. 800 (1940). Although designed to apply especially to the rights of immigrants to shed their foreign nationalities, that Act of Congress ‘is also broad enough to cover, and does cover, the corresponding natural and inherent right of American citizens to expatriate themselves.’ Savorgnan v. United States, 1950, 338 U.S. 491, 498 note 11, 70 S.Ct. 292, 296, 94 L.Ed. 287.

The Supreme Court has held that the Citizenship Act of 1907 and the Nationality Act of 1940 ‘are to be read in the light of the declaration of policy favoring freedom of expatriation which stands unrepealed.’ Id., 338 U.S. at pages 498-499, 70 S.Ct. at page 296. That same light, I think, illuminates 22 U.S.C.A. 211a and 8 U.S.C.A. 1185. Since expatriation is today impossible without leaving the country, the policy expressed by Congress in 1868 and never repealed precludes a reading of the passport and travel control statutes which would permit the Secretary of State to prevent citizens from leaving.

However, it is clear from both immigration and tax law that the U.S. government remains strongly opposed to freedom of emigration today.

Violation of dual citizens’ rights under consular agreements

It’s worth noting that the requirement for dual citizens to use a U.S. passport to enter the U.S. also sneakily circumvents rights that the U.S. is obliged to grant under international treaties. The United States has a number of consular agreements providing that a person who uses a certain passport to enter the country should be treated as a national of that country for purposes of consular protection during the period of his entry. For example, from the exchange of memoranda following the signing of the 1980 Consular Convention with the People’s Republic of China:

All nationals of the sending State entering the receiving State on the basis of travel documents of the sending State containing properly executed entry and exit visas of the receiving State will, during the period for which their status has been accorded, and in accordance with the visa’s period of validity, be considered nationals of the sending State by the appropriate authorities of the receiving State for the purpose of ensuring consular access and protection by the sending State as provided for in Article 35 of the Consular Convention between the United States of America and the People’s Republic of China.

For those still in doubt, Secretary of State Muskie, in his letter to the Senate, was quite clear about the intent:

(3) persons considered to be nationals of both countries will be entitled to the consular protection of the State whose valid passport (properly visaed) they possess while visiting the other

This was hardly unique to China; memoranda exchanged after consular agreements with most other communist countries in 1973 or later contained similar language. The 1972 treaties with Hungary and Romania apparently were not accompanied by such an exchange of memoranda, but treaties the following year with Poland and Czechoslovakia and the year after that with Bulgaria were. These memoranda were generally even more explicit than the Chinese one, stating outright that people using non-U.S. travel documents to enter the U.S. would be treated solely as foreign nationals for the period of their stay “regardless of whether such persons may also be regarded as nationals of the United States of America”. (This leads to the hilarious question of whether such persons should have been subject to U.S. extraterritorial taxation while residing in Czechoslovakia or Bulgaria, but exempt from it while visiting the United States as “non-resident aliens”!)

And lest you think that the 1994 amendment has overridden all of these provisos due to the “last-in-time rule”, note that the 1997 agreement with China regarding the status of the U.S. consulate in Hong Kong after the handover reiterates the presumption that a dual citizen could use either passport to enter the territory of his or her other nationality:

(g) (1) All Chinese nationals of the Hong Kong Special Administrative Region entering the United States on the basis of travel documents of the Hong Kong Special Administrative Region of the People’s Republic of China will, during the period for which their status has been accorded on a limited basis by visa or lawful visa-free entry, be considered Chinese nationals by the appropriate authorities of the United States for the purpose of ensuring consular access and protection by the People’s Republic of China.

(2) All nationals of the United States entering the Hong Kong Special Administrative Region on the basis of travel documents of the United States will, during the period for which their status has been accorded on a limited basis by visa or lawful visa-free entry, be considered nationals of the United States by the appropriate authorities of the People’s Republic of China (including those of the HKSAR) for the purpose of ensuring consular access and protection by the United States.

In practice, the Hong Kong Immigration Department indeed permits Chinese Americans born in Hong Kong to take trips to Hong Kong on their U.S. passports, while the U.S. continues to deny them the right to take trips to the U.S. on their HKSAR passports.

The punishment for violating the law?

The Immigration and Nationality Act of 1952 called for harsh sanctions against people who violated the “valid passport” requirement, stating:

(c) Any person who shall willfully violate any of the provisions of this section, or of any order or proclamation of the President promulgated, or of any permit, rule, or regulation issued thereunder, shall, upon conviction, be fined not more than $5,000, or, if a natural person, imprisoned for not more than five years, or both; and the officer, director, or agent of any corporation who knowingly participates in such violation shall be punished by like fine or imprisonment, or both; and any vehicle, vessel, or aircraft together with its appurtenances, equipment, tackle, apparel, and furniture, concerned in any such violation, shall be forfeited to the United States.

$5,000 in 1952 dollars would be worth $43,919 if adjusted by the CPI, or $210,000 in gold terms — in either case, a fine that could bankrupt an ordinary person — not even mentioning the loss of income, career opportunities, and professional licenses that would result from five years in prison. However, the Foreign Relations Authorization Act for Fiscal Year 1979 had at least one positive impact: it struck out the above section entirely (92 Stat. 993: “Such section is further amended by striking out subsection (c)”).

So now, as Fred and others have pointed out, the law is theoretically toothless: there is no statutorily-authorised penalty against a dual citizen using a foreign passport to enter the United States. If you’re travelling on a Bulgarian, Czech, Slovak, or Hong Kong SAR passport and you managed to get an inattentive Department of State employee to issue you a visa, this would even entitle you to consular protection. However, it’s likely that the above-mentioned consular agreements could not be relied upon to ensure consular protection for an American/Czech or American/Slovak citizen who entered the U.S. under the Visa Waiver Program, since a visa waiver wouldn’t be regarded as a “properly executed visa”. Bulgaria and Hong Kong are not members of the VWP, so this concern does not apply to them; Senator Mazie Hirono (D-HI) has introduced a bill to allow Hong Kong to be considered for the VWP, though some speculate this will face “national security” objections.

However in practice, actually exercising your right as a dual citizen to enter the U.S. on a non-U.S. passport remains extremely difficult, and will probably result in quite a bit of time in the back room with some border guards who have absolutely no idea what the law says. The U.S. embassies in Singapore, Finland, and Argentina all have warnings about § 215(b) on their websites in the sections for dual citizens or reporting births of citizens abroad (note however that none of these countries have China or Bulgaria-like memoranda attached to their consular conventions with the United States); Finland’s, for example, states that:

U.S. citizens who arrive at the port of entry without the required valid U.S. passport may be required to apply for a passport waiver, for which a separate fee may be collected.

Also note that, unlike the 1987 case of the Belgian man whose use of a Belgian passport to enter the U.S. was taken as conclusive evidence of intent to relinquish, these days the use of a non-U.S. passport to enter the U.S. will not likely be sufficient evidence of relinquishment if your other conduct suggests you intend to retain U.S. citizenship.

Practices of other countries

Finally, it’s worth looking at the requirements that other ordinary democratic places impose on their own citizens who may also wish to use foreign passports to enter the country, for whatever reason: none. Such people are simply treated as foreigners for the duration of their stay and can’t exercise certain rights of nationals; for example, here is the law that applies in Taiwan, Republic of China Immigration Act Article 12:

A national without registered permanent residence in the Taiwan Area who has entered the State with a foreign passport or stateless travel document shall exit the State with the foreign passport or the stateless travel document and may not apply for residence or registered permanent residence unless he/she meets the requirements of Subparagraph 3, Paragraph 1 of Article 9 [ed: “Has been naturalized and has acquired the nationality of the State”] or Subparagraph 2, Paragraph 1 of Article 10 [ed: “Was or is born in overseas, resides in the Taiwan Area and has registered his/her permanent residence at a household registry in the area”].

Or from the National Immigration Agency:

A national with Taiwanese household registration and who is currently residing in Taiwan and who has entered Taiwan with a foreign passport and is applying for residency, or an extension of stay or residency, shall first register his/her moving-out registration with the Household Registration Office.

Peruvians on internet forums similarly state that dual citizens can use their foreign passports to enter Peru, though I haven’t been able to find official confirmation of this.