These consolidated appeals arose from the US Coast Guard's interception of a small speed boat in the western Caribbean Sea, and the subsequent arrest and indictment of three men on board for drug trafficking under the Maritime Drug Law Enforcement Act (the MDLEA), 46 USC §§ 70501-08. The appellants challenged the constitutionality of the MDLEA. They petitioned for panel rehearing and en banc review. The Court held their requests in abeyance pending another cognate en banc decision: see United States v Aybar-Ulloa 987 F 3d 1 (1st Cir 2021) (CMI1175). Subsequently, based on the view that Aybar-Ulloa 'diminished the force of this circuit's precedent on the protective principle', a panel rehearing was granted to address the appellants' constitutional challenge to their prosecution under the MDLEA.

In this case, Coast Guard officers boarded and searched the vessel pursuant to a provision of an agreement between the US and Costa Rica. The officers did not find any contraband, but a chemical test detected traces of cocaine. Based on that evidence, the Coast Guard detained the three men - all citizens of Costa Rica - and took them to the US Naval Base at Guantánamo Bay, Cuba, and then eventually to Puerto Rico. At some point, the US contacted the Government of Costa Rica requesting confirmation of the vessel's registry or nationality, and Costa Rica subsequently responded that it could not confirm the vessel's registry. The US thus determined that, pursuant to § 70502(d)(1)(C) of the MDLEA, the boat was 'without nationality', and therefore subject to US jurisdiction. The appellants argued that the MDLEA, particularly § 70502(d)(1)(C), is unconstitutional. In their view, § 70502(d)(1)(C) exceeds Congress's authority under art I of the Constitution, and violates the Due Process Clause of the Fifth Amendment, because it is unconstitutionally vague, subject to arbitrary enforcement, and criminalises conduct that has no nexus with the US.

Held: The appellants' convictions are vacated and the case is remanded to the District Court with instructions to dismiss the MDLEA charges against them.

The question of when a vessel sailing on the high seas may be subject to US jurisdiction under international law - the question at the heart of this case - has constitutional significance. It is a bedrock principle of the international law of the sea that all nations have an equal and untrammelled right to navigate on the high seas: see UNCLOS art 90. To ensure this right of free navigation, international law generally prohibits any country from asserting jurisdiction over foreign vessels on the high seas, and vessels are normally considered within the exclusive jurisdiction of the country whose flag they fly. To preserve this system of flag-State jurisdiction, every vessel must sail under the flag of one and only one State; those that sail under no flag enjoy no legal protection. Although the nationality of a vessel is often referred to as its 'flag', there is no requirement that a vessel fly a physical flag to maintain its nationality. Rather, '[s]hips have the nationality of the State whose flag they are entitled to fly': UNCLOS art 91.1 (Court's emphasis). Therefore, it has long been understood that the US - and any other country - may exercise jurisdiction over vessels that are considered stateless under international law. These general principles of jurisdiction on the high seas are not disputed in this case.

Although the US Senate has not ratified UNCLOS, it was signed by the President, and is generally recognised by the US as reflecting customary international law, ie, universal practice. Moreover, 'many of the provisions of the [UNCLOS] follow closely provisions in the 1958 convention to which the United States is a party and which largely restated customary law as of that time.' The UNCLOS provisions defining a stateless vessel have long been part of the international law of the sea and are largely identical to those in the Convention on the High Seas 1958, which has been ratified by the United States.

Thus, in light of these well-established limitations on Congress's ability to criminalise the conduct of foreign nationals aboard foreign vessels on the high seas, the question that arises when the US seeks to impose its law on foreigners on the high seas is how to identify a vessel that is not within any other country's jurisdiction - potentially exposing those aboard to every country's jurisdiction. In other words, when may a vessel be characterised as stateless?

International law allows each nation to decide for itself the process through which it will grant its nationality to a vessel: UNCLOS art 91.1: 'Every State shall fix the conditions for the grant of its nationality to ships, for the registration of ships in its territory, and for the right to fly its flag.' Authorities encountering a vessel on the high seas would not be aware of some of these circumstances - eg if a state has cancelled a vessel's registration - and thus will be unable definitively to determine nationality by sight even if a vessel is flying a flag. Nonetheless, international law recognises a presumption of nationality in the flag-flying situation, among others. Absent a flag or papers, a vessel may also traditionally make an oral claim of nationality when a proper demand is made. International law also recognises two specific circumstances in which a vessel may be deemed stateless regardless of its actual status, and absent any effort to determine its nationality: when the vessel refuses to claim any nationality, or when it claims more than one nationality: UNCLOS art 92.2: 'A ship which sails under the flags of two or more States ... may be assimilated to a ship without nationality'.

Hence, whether authorities are seeking to ascertain nationality in the first place - by examining documents or eliciting a verbal claim - or to resolve a concern about nationality that was declared by means of a flag, they may need close contact with the vessel and its master. It is therefore understood that international law's so-called 'right of visit' permits authorities to inquire, board, and conduct a limited search designed to elicit information about the vessel's identification and registration.

The MDLEA provides three descriptions for a 'vessel without nationality' in § 70502(d)(1). Two are clearly consistent with international law: when the nation whose registry is claimed denies the claim, and when the individual in charge of a vessel fails to make a claim of nationality or registry for the vessel upon request of an authorised US officer. The third definition, however - the one at issue here - allows a vessel to be treated as stateless where there is a claim of nationality recognised by international law, but the identified country neither confirms nor denies that claim: 46 USC § 70502(d)(1)(C). The Convention on the High Seas 1958 provides that 'each state shall issue to ships to which it has granted the right to fly its flag documents to that effect'. UNCLOS contains a nearly identical provision in art 91.2, and another UNCLOS provision specifically addresses registration, requiring states to 'maintain a register of ships containing the names and particulars of ships flying its flag, except those which are excluded from generally accepted international regulations on account of their small size': art 94.2.a. The US Government suggests that these provisions create an expectation that all vessels will carry documents and that, if a vessel's master does not substantiate a verbal claim with documents or other evidence, the claimed country of nationality 'has accepted through its international treaty obligations that the vessel may be deemed stateless'.

However, these treaty provisions demanding that countries issue documents evidencing vessel nationality say nothing about when a vessel may be deemed stateless. Nor can the provisions reasonably be construed to provide consent to the exercise of jurisdiction over a signatory's vessel by all other signatories based solely on the master's failure to produce documents in support of a claim of nationality. The Government also attempts to infer from treaty provisions a principle of international law that when a country both fails to confirm a claim of registration or nationality, and the vessel carries no registration or other identifying documents, the vessel may be deemed stateless. This theory conflates two discrete international law issues. Even accepting documentation requirements as within customary international law, it does not follow that a country's failure to issue identifying documents or 'maintain a register' renders a vessel stateless when its master has verbally claimed that country's nationality. The relevant question is not whether the claimed country has satisfied its obligations under international law. Rather, the question is what type of inquiry and response suffices to permit the US to deem a vessel stateless, despite a claim of nationality recognised by international law.

Where the master's oral declaration of nationality is consistent with the citizenship or nationality of all individuals aboard the vessel, the declaration is particularly forceful. To reject the master's declaration of nationality in such circumstances, based solely on the claimed country's failure to provide affirmative and unequivocal confirmation - or its failure to respond at all - would eviscerate a method long accepted for identifying a vessel's nationality under international law. One cannot infer displacement of that method merely based on treaty provisions imposing obligations on signatory countries to register vessels or issue other documents.

That is not to say that the Government's emphasis on registration or documentary evidence of nationality is wholly misplaced. International law does, in general, promote a system of registration. It is reasonable to expect that registered vessels would have documents onboard, and, if not, that the claimed country of nationality would be able to easily confirm a legitimate claim by checking its registry. However, not all vessels must be registered. Small vessels are excluded from the UNCLOS registry requirement - see art 94.2.a - perhaps because some countries typically do not register small vessels - whether defined by length or by tonnage. Hence, proof of a vessel's nationality via a centralised registry or other evidence of registration may be unavailable, and a country whose citizens have properly claimed nationality on behalf of their vessels thus may be unable either to confirm or deny those claims when contacted by the US Coast Guard or other authorities.

International law does not require the US to accept a bare assertion of nationality where there is conflicting evidence and attempts to resolve the conflict prove fruitless. Although the master's oral declaration constitutes prima facie proof of nationality, that verbal assertion can be undermined by contrary evidence, as is the case for any prima facie showing. For example, if the vessel's claimed nationality differs from the nationality of most crew members, or if a small vessel is interdicted far from the claimed country, US authorities could properly seek verification of the master's claim. In other words, when US authorities are presented with mixed signals about the nationality of a vessel, it would be permissible under international law for the US to seek confirmation from the country of asserted nationality, and, if none is forthcoming, to treat the vessel as stateless. However, that conflicting-signals limitation is not part of § 70502(d)(1)(C) as currently enacted. The statute on its face is thus inconsistent with international law, and the Court has no licence to rewrite it to satisfy constitutional requirements. It is up to Congress to narrow the language of § 70502(d)(1)(C) if it so chooses.

Even the absence of conflicting evidence of nationality, however, does not mean that foreign nationals engaged in drug trafficking on the high seas can evade prosecution based solely on a verbal claim - whether true or false - of a vessel's nationality. The Coast Guard and other countries' authorities can always ask the claimed country of nationality for consent to arrest and prosecute the individuals onboard. Indeed, it is common practice for countries, including the US, to negotiate bilateral and multilateral agreements to facilitate the apprehension of drug traffickers operating on the high seas. The US also can address its concerns about maritime drug trafficking by seeking to persuade other countries to take enforcement action against their own vessels and nationals. What the US cannot do consistently with the Constitution, however, is arrest and prosecute foreigners on foreign vessels by relying on a concept of statelessness that conflicts with international law. And that is what § 70502(d)(1)(C) allows. It treats as stateless a vessel that, under international law, would be a vessel with nationality. Accordingly, the prosecution of foreign nationals travelling on such a vessel for a violation of US law is impermissible under the Felonies Clause of the Constitution, the only source of authority asserted for Congress's adoption of the MDLEA.