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US submarine sinks Iranian warship off Sri Lanka, marking escalation outside of Persian Gulf

March 04, 2026 5 min read International Relations Internal Security Geography GS2 (International Relations) GS3 (Internal Security Maritime Security)
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What Happened

  • A US Navy submarine torpedoed and sank the Iranian frigate IRIS Dena on March 4, 2026, approximately 40 nautical miles south of Galle, Sri Lanka, in the Indian Ocean
  • IRIS Dena had been participating in Exercise MILAN 2026 and the International Fleet Review hosted by the Indian Navy at Visakhapatnam, and was returning home when attacked
  • US Defense Secretary Pete Hegseth confirmed the attack, marking the first time a US submarine sank an enemy warship using a torpedo since World War II
  • The attack used a Mark 48 torpedo; approximately 180 personnel were aboard — at least 87 were killed, 32 survivors rescued, and over 60 remained missing
  • The incident occurred in the context of escalating US-Israel-Iran conflict following Operation Epic Fury strikes on Iranian military and nuclear targets in late February 2026

Static Topic Bridges

Exercise MILAN — India's Flagship Multilateral Naval Exercise

Exercise MILAN is a biennial multilateral naval exercise hosted by the Indian Navy, first conducted in 1995 with just 4 nations participating. It has grown into one of India's most significant instruments of naval diplomacy in the Indo-Pacific region. The 2026 edition (13th MILAN) was held at Visakhapatnam from February 15–25, 2026, with a record 74 nations, 42 warships and submarines, and 29 aircraft participating under the theme "Camaraderie, Cooperation, Collaboration."

  • MILAN 2026 featured participation from Germany, the Philippines, and the UAE as new entrants
  • The exercise involved harbour and sea phases, with the sea phase (Feb 21–25) covering anti-submarine warfare, air defence, and search-and-rescue drills
  • India showcased indigenous platforms including INS Vikrant (first domestically built aircraft carrier), Visakhapatnam-class destroyers, and Nilgiri-class stealth frigates
  • Iran's participation underscored that MILAN is an open multilateral forum — not an alliance-based exercise — making the subsequent sinking of its frigate particularly diplomatically sensitive for India

Connection to this news: The Iranian frigate IRIS Dena had participated in MILAN 2026 as a legitimate multilateral invitee; its sinking on the return journey raises questions about India's position as host, maritime neutrality, and implications for future multilateral naval diplomacy.

Freedom of Navigation and the Law of the Sea (UNCLOS)

The United Nations Convention on the Law of the Sea (UNCLOS), adopted in 1982 and in force since 1994, governs rights and responsibilities of states regarding the world's oceans. Under UNCLOS, the Indian Ocean beyond territorial waters is "high seas" or "international waters" — no single state may exercise sovereignty. Article 87 guarantees freedom of navigation on the high seas, while Articles 95–96 grant warships complete immunity from the jurisdiction of any state other than their flag state.

  • The attack occurred in international waters (~40 nautical miles south of Galle), well beyond Sri Lanka's 12 nautical mile territorial sea and 200 nautical mile EEZ assertions
  • Attacking a warship of another state on the high seas constitutes an act of war under international law
  • India, as the host of MILAN 2026 that invited Iranian participation, faces reputational questions about the safety of multilateral guests returning through India's maritime neighbourhood
  • Sri Lanka, as the nearest coastal state, was involved in rescue operations at Karapitiya Teaching Hospital in Galle

Connection to this news: The sinking in the Indian Ocean — India's strategic backyard — directly challenges India's concept of "SAGAR" (Security and Growth for All in the Region), its maritime doctrine that emphasises rules-based order and freedom of navigation.

India's SAGAR Doctrine and Indian Ocean Security Architecture

India's SAGAR (Security and Growth for All in the Region) doctrine, articulated by Prime Minister Modi in 2015 at Mauritius, envisions India as a net security provider in the Indian Ocean Region (IOR). It encompasses collective action on maritime security, freedom of navigation, and opposition to disruptive unilateral actions. India's maritime security architecture includes the Information Fusion Centre–Indian Ocean Region (IFC-IOR) at Gurugram, established in 2018, which coordinates maritime domain awareness with over 50 partner nations.

  • The Indian Ocean Region accounts for about 40% of global seaborne trade and 70% of global petroleum trade
  • India's coastline stretches 7,516 km and its EEZ covers over 2.37 million sq km
  • The IFC-IOR uses an information-sharing framework with partner navies, coast guards, and maritime agencies across the Indo-Pacific
  • India is a net security provider under the Colombo Security Conclave (India, Sri Lanka, Maldives, Mauritius, Seychelles, Bangladesh) and the IORA (Indian Ocean Rim Association, 23 members)

Connection to this news: An act of war in international waters 40 nautical miles from Sri Lanka — an Indian Ocean Rim partner — tests India's role as a stabilising force and its diplomatic positioning between the US (a strategic partner) and Iran (a key trade and connectivity partner via Chabahar Port).

Chabahar Port and India-Iran Strategic Connectivity

India has invested significantly in the development of Shahid Beheshti terminal at Chabahar Port in Iran as an alternative trade corridor to Central Asia and Afghanistan, bypassing Pakistan. A 10-year agreement for India to operate the Chabahar Port terminal was signed between India Ports Global Limited (IPGL) and the Port and Maritime Organization of Iran in May 2024.

  • Chabahar is India's gateway to Afghanistan and Central Asia via the International North-South Transport Corridor (INSTC)
  • India-Iran bilateral trade stood at approximately $2.3 billion in FY2024, with Iran being a significant source of petrochemicals and agricultural goods
  • The sinking of IRIS Dena, a vessel Iran commissioned in 2021 as its most advanced indigenous frigate, is likely to severely strain India-Iran relations despite India's non-involvement in the attack
  • India faces the dual challenge of maintaining its strategic partnership with the US (Quad, defence deals) while protecting its Chabahar connectivity and energy import interests linked to Iran

Connection to this news: The attack puts India in a diplomatically delicate position — the attacker (the US) is a Quad partner; the victim (Iran) is a Chabahar partner; and the attack occurred near a Colombo Security Conclave neighbour (Sri Lanka).

Key Facts & Data

  • IRIS Dena: Moudge-class frigate, commissioned 2021; 1,300–1,500 tons displacement, 94 metres length; equipped with Qader anti-ship missiles and a Vertical Launch System (VLS) — first in Iranian Navy
  • Exercise MILAN 2026: 13th edition, 74 nations, 42 ships and submarines, 29 aircraft; held at Visakhapatnam, February 15–25, 2026
  • Mark 48 torpedo: the weapon used; a heavyweight torpedo in service with the US Navy
  • Casualties: ~180 aboard, 87 killed, 32 rescued (treated at Karapitiya Teaching Hospital, Galle), 61+ missing
  • Location: ~40 nautical miles (74 km) south of Galle, Sri Lanka — international waters
  • First US submarine torpedo sinking of an enemy warship since World War II
  • India's SAGAR doctrine enunciated 2015; IFC-IOR established December 2018 at Gurugram
  • UNCLOS: adopted 1982, in force 1994; 168 state parties; India ratified in 1995