The Indian Ocean is the world's third-largest ocean, covering 70.56 million km² between Africa, Asia, and Australia. It carries roughly 80 per cent of global seaborne oil trade and connects to the Strait of Hormuz in the north-west.

Was a warship sunk in the Indian Ocean during the Iran conflict?

Yes. US Defence Secretary Hegseth confirmed on 5 March 2026 that a US submarine sank the Iranian frigate IRIS Dena with a torpedo approximately 40 nautical miles south of Galle, Sri Lanka. It was the first US torpedo sinking of an enemy warship since 1945.Source: Pentagon

Did Iran attack Diego Garcia?

Iran fired two intermediate-range Ballistic Missiles at the joint US-UK base on Diego Garcia during the conflict. One malfunctioned; the other was intercepted. The strike revealed an Iranian missile range of ~4,000 km, double Tehran's previously stated 2,000 km ceiling.Source: UK Ministry of Defence

Background

The Indian Ocean is the post-Hormuz exit corridor for tankers evading Iran's blockade via the Gulf of Oman and the wider Arabian Sea. As of 19 April 2026, Russian seaborne oil continues to transit these waters under GL-134B (extended to 16 May), while Iranian crude loses its equivalent cover under the lapsed GL-U. The divergence is producing a visible split: Russian tankers operating with legal protection eastbound to India and China, Iranian tonnage rerouting to dark-fleet-adjacent Chinese ports. A container ship was damaged by an explosive device 25nm northeast of Oman on 18 April, confirming that the eastern mouth of the Hormuz system remains a kinetic zone.

The Indian Ocean covers 70.56 million km² between Africa, Asia, and Australia, carrying roughly 80 per cent of global seaborne oil trade. The Strait of Hormuz at its north-western entry handles 20 per cent of world oil supply, while Diego Garcia provides the ocean's sole US power-projection base, and the port of Duqm in Oman serves as an auxiliary naval hub.

The Indian Ocean became a live combat zone when a US submarine sank the Iranian frigate IRIS Dena approximately 40 nautical miles south of Sri Lanka on 5 March 2026, the first US torpedo kill since 1945. Iran subsequently fired two intermediate-range Ballistic Missiles at Diego Garcia, 4,000 km distant, shattering its own declared 2,000 km range ceiling. The sinking fell within waters India regards as its strategic sphere. The Russia-Ukraine dimension persists: Russian oil shipments to India cross the same ocean with active US legal cover, while Iranian crude, carried by overlapping dark-fleet tonnage, does not.

More questions

Why is the Indian Ocean important in the Iran war?

The Indian Ocean contains Diego Garcia (the main US regional base), the Strait of Hormuz (20 per cent of world oil supply), and sea lanes critical to Chinese and Indian energy imports. Combat there directly threatens global oil flows and India's claim to regional primacy.Source: Bloomberg

How does the Indian Ocean compare to the Arabian Gulf as a conflict zone?

The Arabian Gulf saw more frequent strikes on ports and naval bases (Manama, Fujairah, Duqm). The Indian Ocean saw the conflict's most significant single naval action: the first US torpedo sinking of an enemy warship since World War II, plus Iranian Ballistic missile strikes on Diego Garcia.Source: event

How the World Sees Them

Iran

Iran contested US naval dominance across both the Gulf and the wider ocean. It fired ballistic missiles at Diego Garcia and claimed hits on a US destroyer, demonstrating reach well beyond its publicly stated 2,000 km missile ceiling.

India

India holds a formal doctrine of Indian Ocean primacy and hosted IRIS Dena at its Fleet Review days before the conflict. The warship's sinking 40 nautical miles from Sri Lanka exposed a gap between India's stated claims and its capacity to enforce them.

Sri Lanka

Sri Lanka occupies a central position in the ocean's northern reaches. It interned the Iranian vessel IRIS Bushehr, declined US and Iranian military access requests, and conducted rescue operations for IRIS Dena survivors.

United States

The US uses Diego Garcia as its primary Indian Ocean base and extends Fifth Fleet command to the western ocean. The torpedo sinking of IRIS Dena was the first offensive submarine kill since World War II.