8 U.S.C. § 1185 215(b) states "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."
Yes, this and its predecessor have been an underlying provision since 1918 or so, ostensibly authorized by the war powers, and I overlooked it . . . but I overlooked it because it has minimal practical impact on ordinary American citizens who are otherwise not in violation of other laws, AND it is overshadowed and subsumed by so much other law. In effect, there are so many exceptions that the exceptions are the rule.
Thus, for example, a U.S. citizen traveler who arrives at a U.S. PoE without a U.S. passport, MUST be allowed to enter the U.S. AND is NOT SUBJECT to prosecution under this provision, regardless the mode of transportation used to arrive at the U.S., with only very NARROW exceptions (the provision has been applied against Americans traveling to Cuba, for example, but the courts have strictly construed its use even in these instances, and to the extent those prosecutions are successful they are also predicated on other more specific prohibitions, such as in the at-the-time applicable Cuba travel ban -- indeed, the overwhelming majority of court cases referencing Section 215(b) of the Immigration and Nationality Act, usually cited as section 1185(b) in title 8 of the United States Code, specifically relate to travel to prohibited countries such as Cuba).
Even the exceptions derived from Presidential Proclamation are also the LAW. Again, the exceptions are so broad and pervasive as to actually be the practical rule.
Additionally, authority has been delegated to the Secretary of Homeland Security to administer other exceptions. The enhanced Drivers License rules, for example, are those issued and published by the Department of Homeland Security, as published in the Federal Register.
Numerous "exceptions" are specifically enumerated in 22 CFR 53.2(b), and 8 CFR 235.1 prescribes restrictions on the scope of border examinations which specify a number of alternative documents (mostly forms of identification other than a passport)
WHICH ARE SUFFICIENT to establish U.S. citizenship and entitlement to enter the U.S.
AND, moreover, however 8 U.S.C. 1185(b) is applied, it CANNOT be applied in a manner which deprives Americans of their constitutional right to travel, and thus the interpretation and application of this statute must accommodate the fundamental right of a U.S. citizen to re-enter the U.S. (as ruled by several federal courts and affirmed by the U.S. Supreme Court). While the current regulations (such as 22 CFR 53.2(b) and 8 CFR 235.1) and rules (which additionally prescribe exceptions, such as those issued by Homeland Security) are comprehensive and cover the field, in significant part they derive from older Court decisions, including those which imposed an injunction on border control officers
compelling them to properly consider alternative documents, that is documents other than a passport, tending to establish U.S. citizenship, which when established requires the border officers to allow a person entry into the U.S.
NEXUS and other "Trusted Traveler Programs:"
As a matter of law, for example, a U.S. citizen and NEXUS card holder presenting a valid NEXUS program card IS
NOT REQUIRED TO BEAR A VALID U.S. PASSPORT. This is specifically prescribed by 22 CFR 53.2(b)(4)(i).
I say this without getting into the proper information and documentation and notice that NEXUS participants need to provide, whether when joining the program or subsequently attendant changed circumstances. I also say this without getting into what is the best practice, or how in practice things tend to go upon arrival at a U.S. PoE. I have been on the downside of a heavy-handed and excessive examination by U.S. border officers, and am well aware that the actual law offers little refuge and less comfort when a U.S. border officer's approach is confrontational . . . in which circumstances even a U.S. passport seems to provide little cover.
In any event, in particular, 22 CFR 53.2(b)(4) (CFR means "Code of Federal Regulations") specifically provides that
a U.S. citizen is NOT required to bear a valid U.S. passport to enter or depart the United States when properly used pursuant to various trusted traveler programs, including NEXUS, FAST, and SENTRI.
FURTHER CAVEAT:
In general, a U.S. citizen should declare his or her U.S. citizenship upon arrival at a U.S. PoE, and present either a U.S. passport or one of the specifically provided alternatives, such as a U.S. State enhanced Drivers License . . . or a valid NEXUS card, for example . . . and there are additional options for children as well.
In general, the failure to present proper U.S. documentation upon seeking entry can often, if not usually, result in a referral to Secondary and a stern admonition, along with some largely vacuous threats (largely vacuous since, again, despite 8 U.S.C. 1185(b), a U.S. citizen without a U.S. passport cannot be prosecuted for seeking entry to the U.S., unless there is some other violation of law also involved). This encounter can be unpleasant and entail a rather inconvenient delay. But the law is not broken. There are no grounds for making an arrest or bringing charges or unnecessarily detaining the American (none based on not having a U.S. passport that is).
Recognition of other citizenship:
"Recognition" in this context is one of those terms which has a different meaning when used as a legal term versus its common usage in general. Sure, U.S. authorities will acknowledge or admit awareness of a person's other citizenship, and in general is very well aware that many U.S. citizens also have citizenship in other countries. But this is not about recognizing the individual's other citizenship in a legal sense.
In particular, the U.S. does not recognize a U.S. citizen's other citizenship as legal status. Thus, for example, a dual U.S. and Canadian citizen has NO right to ask for Canadian consulate assistance when detained by U.S. law enforcement.
The status of a dual Canadian and U.S. citizen is the same, in the individual's dealing with U.S. authorities, as any other U.S. citizen.
And it is largely likewise in Canada, Canada generally acknowledging many Canadians are also citizens of other countries, but so far as Canadian authorities are concerned, a Canadian citizen with another citizenship has no different status in Canada as any other Canadian citizen (for a brief period of time, pursuant to Harper's Bill C-24, there was a distinction in certain contexts relative to potential revocation for criminal acts, but that was repealed by Trudeau's Bill C-6).