Who owns the Strait of Hormuz?

Iran has significantly restricted or completely stopped shipping traffic through the Strait of Hormuz since the United States and Israel launched attacks on Iranian territory on February 28. Prior to the war, around 20% of global oil and seaborne gas shipments passed through the strait, and the subsequent impact on worldwide energy resources and economic activities has prompted ongoing efforts to resolve the standoff. As rising energy prices increasingly affect individual consumers, establishing baseline knowledge about the legal issues at play in the strait can help observers better understand the complexities of negotiations between Iran and the United States.

• Within the Strait of Hormuz, Iran and Oman each claim 12 nautical miles from their coastlines as their territorial waters, with all the legal and political rights associated with that status. At its narrowest point, the strait is only 21 miles wide, making it impossible for ships to transit the strait while remaining in international waters.

• In 1971, Iran seized control of the islands of Abu Musa and Greater and Lesser Tunb. These islands allow Iran to claim even more of the strait and deploy military resources farther from its mainland, despite unresolved claims on the islands by the United Arab Emirates.

Lawful passage through strategically important maritime routes is not only determined by individual governments’ claims of ownership or control. A body of international law and established customs govern the usage of key waterways around the world, with the most recent major multilateral agreement being the United Nations Convention on the Law of the Sea (UNCLOS). UNCLOS has been in effect since 1994 but has not been ratified by—and therefore is not enforceable against—the United States or Iran. The strategic importance of the Strait of Hormuz and longstanding antagonism between Iran and other countries worldwide mean debates over the lawful use of the strait pre-date the current conflict and are likely to remain relevant well into the future.

• UNCLOS distinguishes between the rights of transit passage and innocent passage in determining the lawful use of international straits. It requires the Strait of Hormuz be governed by the standard of transit passage, in which ships must be allowed unimpeded travel, if that travel is “continuous and expeditious”, and not that of innocent passage, which requires unimpeded passage unless ships may threaten the peace or security of the country whose territorial waters they are transiting.

• Although the United States is not a party to UNCLOS, the longstanding US position is that transit passage is a recognized custom that should be applicable even to non-parties to UNCLOS. This standard suggests that the right of transit passage through the Strait of Hormuz is so widely accepted and well-established that it has taken on the force of international law that transcends the agreement in which it was officially established.

• Iran has cited pre-UNCLOS international law that favors the application of innocent passage within its territorial waters, including the strait, and gives Iran the authority to determine whether the transit of any individual ship is actually innocent or should be stopped. If applied to current events, the standard of innocent passage might allow Tehran to stop ships that enter its territorial waters, although the deployment of sea mines and military force against those ships would not be permitted.

• Iran and the US, as non-partners to UNCLOS, are very unlikely to this crisis through appeals to international maritime law. The reopening of the Strait of Hormuz will almost certainly be part of a larger agreement involving issues beyond the strait but will be shaped by each side's understanding of relevant definitions and standards.