An expert's point of view on a current event.

Why the United States Can’t Touch Iran’s Atlantic-Bound Warships

International law blocks direct action on sovereign vessels even if they’re violating U.S. sanctions.

By , an associate research analyst at CNA.
An Iranian warship nears the Strait of Hormuz.
An Iranian navy warship nears the strategic Strait of Hormuz in southern Iran on Sept. 11, 2020. Iranian Army Office/AFP via Getty Images

Last week, Politico reported the movement of two Iranian warships apparently on their way to the Atlantic Ocean. U.S. national security officials expressed concern that these ships were bound for Venezuela with cargoes that violate U.S. sanctions on Caracas. Already, Sen. Marco Rubio has called for the United States to prevent the ships’ arrival. However, any U.S. action against these vessels would be unlawful and undermine a core tenet of the international order: sovereign immunity. The costs of direct action would be severe, exposing the United States to charges of hypocrisy toward the rules-based order and potentially opening U.S. naval vessels to similar treatment by adversaries.

Caracas and Tehran have grown close over the past decade as each has found relief in the other as a safety valve from U.S. sanctions. Trade in oil has been particularly important for the duo, and the United States and its allies have, in recent years, interdicted several cargo vessels under flags of convenience suspected of ferrying Iranian oil in violation of U.S. and European Union sanctions. This time is different. These vessels are part of the Iranian navy. Under international law, Tehran can channel rapper MC Hammer and tell the United States, “you can’t touch this.”

The law of the sea, whether customary or conventional, grants warships and other government ships sovereign immunity. In times of peace, sovereign immunity is a practically all-powerful ward against a foreign state’s jurisdiction. Exceptions may apply in extreme circumstances involving failed states, fake warships, or weapons of mass destruction. This case, however, is textbook.

The U.N. Convention on the Law of the Sea (UNCLOS) defines warships as vessels “belonging to the armed forces” under the command of an officer in the service list and manned by a crew in good order. Both Iranian vessels, an unnamed frigate and the IRINS Makran, both clearly meet the definition of a warship under the U.N. convention.

The U.N. Convention, which the United States believes reflects customary international law, explicitly spells out some of sovereign immunity’s power. And on the high seas, sovereign immunity is absolute. Article 95 simply reads: “Warships on the high seas have complete immunity from the jurisdiction of any State other than the flag State.” Article 96 provides the same absolute immunity to government-owned or operated vessels on the high seas. This right also applies in exclusive economic zones (EEZ) since nothing in that section overrules the provisions.

Even in the territorial sea, sovereign immunity remains a powerful protection. Warships enjoy the right of innocent passage in foreign territorial seas. The coastal state may establish rules for navigational safety but has essentially no power to enforce these regulations on foreign warships that flout traffic separation schemes or the like. As long as the warship is engaged in innocent passage, not threatening the coastal state, the coastal state can, at most, order the warship to leave the territorial sea. Interdiction or arrest are out of the question unless the warship threatens the coastal state, at which point self-defense would be permitted.

Internal waters, such as ports, are not substantially different. A warship would, of course, need the coastal state’s permission to enter internal waters. Yet even here, where the coastal state has its greatest authority, sovereign immunity retains its power under well-accepted, customary international law. The International Tribunal for the Law of the Sea (ITLOS) affirmed this in 2012’s “ARA Libertad” case. The Libertad is an Argentinian naval vessel that called at Tema, a Ghanaian port, in late 2012. As part of enforcing a U.S. court order over unpaid Argentine sovereign debts, Ghanaian courts ordered the vessel to remain in the harbor, and Ghanaian authorities attempted to board the vessel. In December 2012, ITLOS’s judges unanimously ordered Ghana to release the vessel immediately. The majority reasoning noted “a warship is an expression of the sovereignty” of its flag state and “in accordance with general international law, a warship enjoys immunity, including in internal waters.”

ITLOS judges Rüdiger Wolfrum and Jean-Pierre Cot built an even more robust argument in their concurring opinion. There, they took issue with the majority’s summary reasoning and instead closely examined both proposals relating to internal waters and warships prior to UNCLOS and the language of that convention itself. They ultimately concluded “that warships in internal waters enjoy immunity from the exercise of coastal state jurisdiction, which includes immunity from judicial proceedings or any enforcement measure, [and] is well established in customary international law.” This principle, the judges noted, was recognized not only by the Institut de Droit International as early as 1898 and again in 1928 but also by various national court cases, including the U.S. Supreme Court’s Schooner Exchange v. McFaddon and Others.

Nothing changes even if U.S. officials ascertain the vessels are carrying conventional arms that violate U.S. sanctions on Caracas. Consider U.N. Security Council-endorsed sanctions on North Korea. That system includes perhaps the most robust system of sanctions on ship-borne materials and comes with the backing of the five permanent Security Council members (even if China’s enforcement has been lukewarm). Although silent on sovereign immune vessels, the most recent resolution in 2017 allowed member states to “seize, inspect, and freeze any vessel subject to its jurisdiction in its territorial waters.” Since customary international law holds that warships during peacetime are never under the jurisdiction of a foreign state, foreign warships are arguably not subject to foreign enforcement actions under these U.N. sanctions.

In this case, so long as the Iranian warships do not threaten use of force, sovereign immunity protects them wherever they are—whether in the high seas, an EEZ, a territorial sea, or internal waters. If conventional arms are aboard, the U.S. enforcement action would merely rest on national sanctions while even the most robust U.N. sanctions may not cover sovereign immune vessels. Likewise, the “ARA Libertad” precedent clearly demonstrates even if these vessels are forced to request and receive permission to call at a port to replenish their supplies, the United States gains no legal options. The port state remains bound by customary international law.

The United States could ignore the law, as Iran does in the Persian Gulf, but this comes with significant costs. If an attempted enforcement action leads to a U.S. loss before an international court, the United States suffers a humiliating defeat that may embolden Iran. If the enforcement action succeeds both operationally and legally, the United States could put U.S. naval vessels at jeopardy, if, say, China decided U.S. naval vessels supplying arms to Taiwan violated future Chinese sanctions.

A success or failure, U.S. action directly against the Iranian vessels steaming across the Atlantic Ocean will complicate efforts to secure U.S. interests and position the United States as a champion of the international rules-based order. To prevent Iran’s naval vessels from reaching Venezuela and to advance U.S. interests, the United States should employ diplomacy rather than force and encourage states along the route to deny the Iranian vessels port access if requested. But policymakers and elected officials itching to send in the U.S. Navy or Coast Guard would do well to remember MC Hammer’s basic rule of sovereign immunity: “You can’t touch this.”

Cornell Overfield is an associate research analyst at CNA. The views expressed here are his own and do not reflect his employer.