When Sanctions Become Seizure: The Law of the Sea on Trial

In December 2025, successive interdictions in the Caribbean have laid bare a hard problem for the modern law of the sea. What began as United States enforcement against designated Venezuela linked shipping is now reported to have moved into behaviour based boardings of vessels not publicly designated, forcing the question that matters: does freedom of navigation still operate as rule, or is it being recast as discretion.

What happened, as reported

On 20 December 2025, the United States Coast Guard boarded the crude oil tanker Centuries in international waters in the Caribbean. Multiple outlets reported that the vessel was Panama flagged at the time and was intercepted east of Barbados. Reuters and other reporting further stated that the tanker was carrying roughly 1.8 million barrels of Venezuelan Merey crude and that the shipment was bound for China through intermediaries.

Public reporting also stated that neither Centuries nor its Hong Kong based owner appeared on published United States sanctions lists at the time of the operation. That point is not decoration. It is the fulcrum.

As to method, United States officials described the operation as a boarded inspection conducted with flag state involvement. Reuters reported that Panama was said to have approved the boarding. The Washington Post reported that United States officials cited a maritime law basis for boarding suspicious vessels and that Panamanian approval was obtained. Video of the operation was released on social media by Homeland Security Secretary Kristi Noem and carried widely by broadcasters, showing an airborne insertion onto the ship’s deck.

Panama’s foreign minister, Javier Martinez Acha, stated in a televised interview on 22 December that the vessel had violated Panama’s maritime regulations by switching off its transponder while departing Venezuelan waters, adding that Panama would take measures in response. Reuters reported that flag states have the authority to revoke registration where their maritime rules are breached.

The Centuries operation followed earlier United States action against other vessels linked to Venezuelan oil flows. Reporting by major outlets describes a December seizure of the tanker Skipper framed as a sanctions and law enforcement matter, including reliance on judicial process in at least some instances. Separate reporting, including by the Financial Times and the Associated Press, described United States pursuit of a third tanker, Bella 1, characterised as sanctioned and allegedly operating under a false flag.

The distinction between a designation led seizure, supported by a judicial instrument, and a behaviour led boarding of a vessel not publicly designated, is decisive. That is the difference between law constrained enforcement and discretionary interdiction.

How the sides are framing it

Washington presents the operations as enforcement against a Venezuelan shadow fleet, alleging evasion tactics, identity masking, and illicit flows of PDVSA linked crude. Reuters reported United States claims that Centuries had used the false name “Crag” and that the cargo involved intermediaries supplying Chinese buyers. Reuters also reported United States characterisations linking the trade to narco terrorism funding.

At the political level, the rhetoric has been sharper than the law. Reuters, the Washington Post, and other outlets reported President Trump describing a “total and complete blockade” of sanctioned oil tankers entering or leaving Venezuela. The word blockade matters, because it is a legal term of art even when used as a political slogan.

Venezuela has described the actions as piracy and theft, and has signalled recourse to the United Nations and other international bodies. Those statements have been carried in Reuters and other coverage.

China has responded in explicitly legal terms. Reuters reported the Chinese foreign ministry calling the seizure of ships a serious violation of international law, condemning unilateral sanctions, and stating that Venezuela’s right to partnership should be respected. That framing is deliberate. It is aimed at precedent, not sympathy.

The law the incident engages

The governing framework is orthodox. In peacetime, freedom of navigation on the high seas is the default condition. UNCLOS Article 87 expresses the principle. UNCLOS Article 92 anchors exclusive flag state jurisdiction on the high seas.

Interference with a foreign flagged merchant vessel is exceptional. The right of visit under UNCLOS Article 110 is narrowly drawn. It applies to piracy, slave trade, unauthorised broadcasting, statelessness, or cases of genuinely grounded doubt as to nationality. Behavioural suspicion, including AIS darkness, does not appear in that list.

A flag state may consent to boarding by a third state. Such consent is legally effective, but it is not transformative. It cannot, by repetition, convert a narrow cooperation mechanism into a general enforcement jurisdiction over foreign commerce. There is an additional complication here. The United States and Panama have long had bilateral maritime arrangements facilitating rapid authorisation for certain operations. The United States State Department has published a 2002 supplementary arrangement on maritime operations signed in Panama City. The existence of a streamlined consent mechanism may explain operational speed. It does not, by itself, answer the legality question if the practice becomes a routine substitute for the treaty limits in UNCLOS.

Blockade, properly understood, is a belligerent measure governed by the law of armed conflict. Outside armed conflict, and absent Security Council authority, a unilateral blockade language sits uneasily with the Charter framework. Even if the United States is not claiming a blockade in the legal sense, repeated reliance on the term risks importing legal consequences and legal challenge.

Where legality turns

The legality analysis must separate two questions that political commentary tends to merge.

First, the boarding act. If Panama provided clear and specific consent to board a Panama flagged vessel, the boarding itself may be capable of narrow technical justification as a consent based inspection. On the public record, that is the strongest point available to the United States, and it is the only one that does not require rewriting UNCLOS.

Second, the underlying purpose and method. A behaviour led interdiction of a vessel not publicly designated, justified by suspicion markers and routed through consent mechanisms, is legally vulnerable. AIS darkness may be a breach of flag state rules. It is not, without more, a recognised high seas visit ground under Article 110. If the practical aim is to deter or disrupt third party trade beyond designation and beyond judicial process, then the interference approaches coercion rather than orthodox enforcement.

That is why the Skipper comparison matters. A designation led operation, supported by a judicial instrument, is legally intelligible even when contested. A behaviour led boarding of an undesignated vessel expands the contestable category from those identified by law to those rendered vulnerable by allegation.

What follows, and why it matters

This is not a marginal episode. It is a methodological shift.

If interdiction becomes behaviour based rather than designation based, any flow can be made contestable. The precedent matters more than this cargo. It moves the legal centre of gravity away from treaty predicates and toward discretionary enforcement. That is precisely what China is trying to prevent from becoming normal.

On the facts presently reported, the Centuries boarding may admit a narrow justification through flag state consent and a stated inspection rationale. But the broader practice, targeting undesignated third party trade absent a recognised Article 110 predicate, absent a judicial instrument comparable to warrant led seizures, and absent Security Council authority, is difficult to reconcile with the law of the sea as conventionally understood.

That is how maritime order is lost.