Prof. James Kraska: “The US Submarine Attack on the IRIS Dena Complied with the Law of Naval Warfare”

Following the sinking of the IRIS Dena, some critics claimed the attack on the Iranian warship was illegal.  That struck me as plainly wrong, but I decided to consult one of the most respected experts on naval warfare law, Dr. James Kraska (see his awesome bio!).  Among other things, he was key in the publication of the 2025 edition of the Newport Manual on the Law of Naval Warfare, the world’s foremost treatise on the topic.

My bet is that you will find quite a bit about the facts and law in Dr. Kraska’s article that you haven’t seen or heard in many news reports about the sinking.  For example, as James explains, the “IRIS Dena was a lawful military objective, regardless of whether it posed an imminent threat to U.S. forces or was operating beyond the theater of combat operations.”

A quick note: the IRIS Dena was a formidable warship.  As one source describes it:

“IRIS Dena is a Mowj class frigate utilized primarily for power projection. Armament includes 4 Qader anti-ship missiles, 2 x Sayyad air-defense missiles, various guns including a 76 mm main gun, a 40 mm gun and two 20 mm and 12.7 mm guns. The ship is equipped with lightweight torpedoes and a 3D phased array radar.”

With scores of U.S. Navy ships battling Iran, and with Iran trying to disrupt the flow of energy supplies by sea that are vital to the world’s economy, it shouldn’t be surprising that the U.S. has sunk over 30 Iranian warships.   Before anyone gives any credence to allegations that the U.S. military has violated the law of war in these sinkings, it is imperative that the facts and the law be understood.  James’ essay helps us do just that. 

The US Submarine Attack on the IRIS Dena Complied with the Law of Naval Warfare

James Kraska*

On March 4, 2026, a U.S. Navy fast-attack submarine sank the Iranian frigate IRIS Dena in the Indian Ocean, approximately 19 nautical miles off the southern coast of Sri Lanka near the port of Galle. Casualty figures vary slightly across reports: at least 87 Iranian sailors were killed, with Sri Lankan authorities reporting they rescued 32 survivors.

U.S. Secretary of Defense Pete Hegseth confirmed the incident during a Pentagon press conference the following day, stating: “An American submarine sank an Iranian warship that thought it was safe in international waters. Instead, it was sunk by a torpedo. Quiet death.”

The IRIS Dena was offered safe harbor by India at one of its ports prior to the attack, but the offer was declined. Another Iranian vessel, IRIS Lavan, docked in Kochi on March 4 and was spared from destruction. The attack provides an opportunity to consider the modern application of the law of naval warfare, which is accurately restated in The Newport Manual (Second Edition).

The Fourth Submarine Attack against a Warship Since WW II

The attack was the first time a U.S. submarine had sunk an enemy warship with a torpedo since World War II, and the first combat engagement by a U.S. nuclear-powered attack submarine. This was only the fourth time that a submarine sank a warship since World War II.

On December 9, 1971, during the Indo-Pakistani War, the Pakistani submarine PNS Hangor torpedoed and sank the Indian anti-submarine frigate INS Khukri, resulting in the loss of 194 Indian sailors. The INS Khukri had detected and engaged the PNS Hangor, which had fired a torpedo at another Indian Navy warship, the INS Kirpan. This was the first submarine attack on a warship since World War II.

During the 1982 Falklands War, the British nuclear-powered submarine HMS Conqueror torpedoed and sank the Argentine cruiser ARA General Belgrano on May 2, 1982. The Belgrano was Argentina’s largest warship, with a crew of 1,048 men. Amazingly, resuce efforts by Argentina and Chile rescued 800 sailors, despite 18-foot seas and 75-mile an hour winds and temperatures that plunged to 20 degrees C below zero.

After 323 crew members were lost, the Argentine Navy withdrew its surface fleet from further operations. On March 26, 2010, the ROKS corvette Cheonan was sunk by a torpedo, with the loss of 46 of the 104 sailors on board. A team of investigators from Sweden, Australia, Britain, and the United States attributed the sinking to a torpedo launched by a North Korean submarine.

The Law of Naval Warfare

The attack was conducted in accordance with the law of naval warfare. The laws of naval warfare at sea are distinct and sometimes different than the law of armed conflict on land. The rules governing attacks by submarines in the law of naval warfare are essentially the same as those that apply to surface warships and military aircraft. (Newport Manual 8.1.3)

The law of naval warfare comprises the rules applicable during international armed conflict at sea. Iran and the United States are in an international armed conflict. Common Article 2 of the Geneva Conventions states that the laws of war “shall apply to all cases of declared war or of any other armed conflict which may arise between two or more of the High Contracting Parties, even if the state of war is not recognized by one of them.”

Lawful Military Objective

Enemy warships, including frigates like the IRIS Dena, qualify as legitimate military objectives under the principle of distinction, regardless of whether they are actively fighting, as long as they are capable of making an effective contribution to military action. The IRIS Dena was a lawful military objective, regardless of whether it posed an imminent threat to U.S. forces or was operating beyond the theater of combat operations.

Although the United States is not party to Additional Protocol I of the Geneva Conventions, article 52(2) defines a military objective as: “those objects which by their nature, location, purpose or use make an effective contribution to military action and whose total or partial destruction, capture or neutralization, in the circumstances ruling at the time, offers a definite military of advantage.”

The ship’s participation in recent exercises and location far from the primary conflict zone do not alter its status as a target. The fact that the ship was not actively engaged in combat is immaterial to its status as a lawful target.

International Waters

The IRIS Dena was a Moudge-class frigate approximately 312 feet long that recently had departed India after participating in a multinational naval exercise in the Bay of Bengal. The warship was bound for the Strait of Hormuz just after Iran vowed to attack any vessel traveling through the strait. Furthermore, the U.S. attack occurred in what are accurately described as “international waters.”

In accordance with article 58 of United Nations Convention on the Law of the Sea and the law of naval warfare, the attack occurred in the exclusive economic zone (EEZ) of Sri Lanka, an area where high seas freedoms of navigation and overflight and other internationally lawful uses of the sea apply. Belligerent armed forces are entitled to conduct hostilities in the EEZ under the law of naval warfare.

Before the attack, U.S. forces issued two warnings to abandon the ship. The commander refused the warning, which led to an argument among the crew. The law of naval warfare does not require warning an enemy surface ship before engagement and destruction. Different rules, however, apply to enemy merchant ships.

The 1936 London Protocol requires submarines to follow the same rules of international law as surface warships when acting against merchant ships. This means a warship (surface or submarine) may not sink or disable a merchant vessel without first placing passengers, crew, and ship’s papers in a place of safety—except in cases of persistent refusal to stop when summoned or active resistance to visit or search.

These “cruiser rules” (or prize rules) aimed to protect civilian merchant shipping by requiring submarines to surface, warn, search, and provide for the safety of crews before sinking vessels. But these rules apply only to merchant ships and do not restrict attacks on enemy warships, which may be sunk without warning under customary naval law.

Search and Rescue

Geneva Convention GC II (article 18), parties to a conflict must, after each engagement at sea, take all possible measures without delay to search for and collect the shipwrecked, wounded, and sick (including those from an enemy warship sunk by a submarine), to protect them, ensure their care, and search for the dead.

However, this obligation is not absolute and is conditioned on what is practically and operationally feasible. Submarines are not required to surface or conduct direct rescue operations if doing so would endanger the submarine, such as by compromising its stealth, position, or safety. Due to limited space and the special vulnerability when surfaced, there are what might be termed common but differentiated responsibilities on submarines compared with surface warships, which are recognized in state practice of naval combat. The ICRC Commentary to GC II, for example state (para. 1642 – 1643):

1. 1642. The question of a vessel’s operational capacity arises with particular significance in the case of submarines (and even more so in the case of unmanned naval systems). As a matter of international law, submarines are bound by the same rules as surface vessels, and Article 18 is no exception. In practice, of course, space is extremely limited on board a submarine, thus complicating their ability to take on board shipwrecked, wounded and sick, let alone dead, persons. Thus, several commentators, using different formulations, concur that a submarine may not be required itself to surface in order to carry out a search and rescue operation, for which it may be ill-equipped to begin with. This does not mean, however, that a submarine, which, for example, has successfully torpedoed an enemy warship, would not be required to assess what other measures may be ‘possible’ for it to undertake.

1. 1643. Provided doing so does not render the submarine detectable to the enemy …, Article 18 may require its commander to alert his or her own authorities and, where possible, other entities, to the location of the attack and to the possibility that there may be survivors, thereby allowing the Party to the conflict to assess which ‘possible measures’ may be taken, for example sending other vessels to the area. While doing so may not be possible seconds after launching its weapon, it may become feasible once the submarine has moved away from the area.

The United States complied with its obligations by swiftly contacting Sri Lankan authorities after sinking the IRIS Dena, satisfying the duty without direct recovery by the attacking submarine itself.

The U.S. contacted Sri Lankan authorities after the attack and provided the location for search and rescue. Sri Lanka rescued 32 personnel from the sinking frigate. Hospital authorities in Galle said 87 bodies were brought in by military rescuers. Around 60 of the estimated 180 on board were likely lost at sea.

Conclusion 

The U.S. Navy’s torpedo attack on the Iranian frigate IRIS Dena on March 4, 2026, fully complied with the established rules and principles of the law of naval warfare. The attack demonstrates enemy warships remain lawful military objectives in an international armed conflict irrespective of their immediate actions.

The incident underscores that parties to a conflict freely conduct belligerent operations in neutral state exclusive economic zones. Participation in multinational exercises does not immunize a warship from attack.Unlike merchant vessels subject to the 1936 London Protocol’s cruiser rules requiring warning and provision for safety, enemy warships like the IRIS Dena may be engaged without prior warning under customary international law.

In this case, however, U.S. forces issued warnings before the strike, although they were not legally required to do so. The refusal to abandon ship was a fateful and cruel error by the captain of the IRIS Dena, but it did not alter the vessel’s targetable status.

Post-attack obligations under Geneva Convention II were met through prompt notification to Sri Lankan authorities, enabling the rescue of survivors and recovery of sailors lost in combat. There is no expectation that the attacking submarine surface or directly recover personnel given operational constraints on stealth, space, and safety.

This incident, the first U.S. submarine sinking of an enemy warship since World War II, underscores the enduring distinction between naval and land-based rules of armed conflict, while affirming that belligerents retain broad latitude to destroy legitimate military objectives far from primary theaters.

Finally, the rules governing attacks at sea reflect a looser understanding of the application of the principles of distinction, proportionality, and precautions in attack because the distributed nature of war at sea means there is substantially lower risk of injuring or killing civilians.

About the author 

* James Kraska is Charles H. Stockton Professor of International Maritime Law and Chair of the Stockton Center for International Law at the U.S. Naval War College and Visiting Professor of Law and John Harvey Gregory Lecturer on World Organization at Harvard Law School. He has served as a visiting professor of law at the University of the Philippines and Gujarat National Law University as a visiting scholar at Duke University Marine Laboratory and Woods Hole Oceanographic Institution.

Professor Kraska is a retired U.S. Navy judge advocate, with multiple tours of duty in Japan and the Pentagon, including as Oceans Law & Policy Adviser as well as Director of International Negotiations on the Joint Staff J-5. His most recent books are Marine Technology, Ocean Development and the Law of the Sea (with Lagdami, Cambridge University Press 2026) and Cultural Influences on the Law of the Sea (with Ellison, Oxford University Press 2025). He also leads the publication of The Newport Manual on the Law of Naval Warfare and The Commander’s Handbook on the Law of Naval Operations. He is a Lifetime member of the Council on Foreign Relations.

Dr. Kraska holds a J.D. from Indiana University Maurer School of Law, an LL.M. from University of Virginia School of Law, and an S.J.D. from University of Virginia School of Law.

* Disclaimers:

The views presented are those of the author and do not necessarily reflect the official policy or position of the U.S. Naval War College, the Department of the Navy, or the Department of War.

The views expressed by guest authors do not necessarily reflect my views or those of the Center on Law, Ethics and National Security, or Duke University. (See also here).

Remember what we like to say on Lawfire^®: gather the facts, examine the law, evaluate the arguments – and then decide for yourself!