In late February, Iran’s Islamic Revolutionary Guard Corps (IRGC) moved to close the Strait of Hormuz, issuing warnings that “if anyone tries to pass, the heroes of the Revolutionary Guard and the regular navy will set those ships ablaze.” By targeting ships with drones and antiship missiles and laying mines, Tehran has choked off oil exports from the Middle East and sent energy prices soaring.

The control of waterways has long been used to thwart adversaries and shape strategic outcomes. In 1951, after Tehran nationalized its oil industry, the United Kingdom used naval pressure to prevent Iran from exporting oil. During the 1984 “tanker war,” Iran laid mines in the Strait of Hormuz and harassed ships in response to Iraqi attacks. Throughout both these conflicts, however, the Strait of Hormuz remained in use.

What the current Hormuz crisis makes plain is that closing a strait has become easier and the consequences more far-reaching. Relatively inexpensive technologies—including coastal surveillance systems, shore-based antiship missiles, drones, uncrewed surface vessels, and mines—now allow weaker states to disrupt at scale and impose costs on stronger adversaries. At the same time, the concentration of global trade and energy flows through a handful of narrow routes has magnified the impact of localized crises. Critically, the U.S. and Israeli strikes on Iran and U.S. President Donald Trump’s subsequent threat to blockade the Strait of Hormuz suggest a greater willingness by major powers to impose extensive economic costs and disregard international law, including the rules governing transit. A waterway, moreover, need not actually be closed to cause extensive pain: the mere threat of disruption is enough to raise insurance premiums, reroute shipping, and unsettle commodity markets.

For Asia, the stakes are higher still: whereas Hormuz is largely an energy chokepoint, Asia’s waterways sit astride global trade, energy, and semiconductor supply chains. By demonstrating both the feasibility—even for a weaker power—of weaponizing a chokepoint and the willingness of powerful states to impose and tolerate widespread costs, Hormuz could encourage similar tactics across the Indo-Pacific. This could take the form of U.S. restrictions on access through the Strait of Malacca, a Chinese blockade of the Taiwan Strait, or U.S.-Philippine denial of access through the Luzon Strait.

Pressure on these primary chokepoints could in turn extend to secondary waterways. Recent developments in the Indonesian archipelago, which have largely gone under the radar, suggest that both Washington and Beijing are more keenly anticipating disruptions and maneuvering to contest Asia’s secondary maritime corridors.

THROWING OUT THE RULEBOOK

After U.S. and Israeli strikes in late February, Iran did not merely retaliate with kinetic attacks. The IRGC also established a highly structured toll system in the Strait of Hormuz that required vessels to submit documentation and pay fees in order to pass. While unconfirmed reports emerged of at least one vessel paying $2 million to transit the strait, many shipping companies have publicly insisted that they will not pay fees, citing the “principle of navigation based on international law.”

In mid-April, after negotiations to end the war and reopen the strait broke down, Trump declared the U.S. Navy would blockade “any and all Ships trying to enter, or leave, the Strait of Hormuz.” The declaration raised immediate legal concerns under both the UN Convention on the Law of the Sea and the law of armed conflict at sea, which governs naval warfare and conduct toward neutral shipping. For straits that are 24 nautical miles wide or narrower, such as the 21-nautical-mile-wide Strait of Hormuz, the regime of “transit passage” under UNCLOS grants ships and aircraft the right of unimpeded navigation and overflight. Although the United States has not acceded to UNCLOS,........

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