On Oct. 31, 1803, the frigate USS Philadelphia hit a reef and ran aground chasing Barbary pirates near Tripoli, Libya. All 307 members of its crew were captured and held hostage by the Barbary states. Rather than pay tribute to the pirates, U.S. President Thomas Jefferson sent U.S. frigates to intervene and free the hostages. The first of two wars against the Barbary pirates, the scourge of shipping for decades, hastened the building of the U.S. Navy and began a long tradition of enforcing freedom of navigation—keeping the seas open to all commercial traffic—that has been a vital interest and core principle of U.S. foreign policy. Until now.

Two hundred and twenty-three years later, Iran’s attempted monetization of the Strait of Hormuz, implicitly endorsed by the United States in the initial memorandum of understanding (MOU) and shortly followed by U.S. President Donald Trump’s rival plan to charge a 20 percent fee on all shipping through the strait (though he withdrew the proposal the next day), may be the death of that principle. Neither the United States nor Iran have ratified the U.N. Convention on the Law of the Sea, whose Article 38 codifies commercial ships’ freedom of transit passage, while Article 44 compels states bordering straits not to hamper or suspend that passage.

On Oct. 31, 1803, the frigate USS Philadelphia hit a reef and ran aground chasing Barbary pirates near Tripoli, Libya. All 307 members of its crew were captured and held hostage by the Barbary states. Rather than pay tribute to the pirates, U.S. President Thomas Jefferson sent U.S. frigates to intervene and free the hostages. The first of two wars against the Barbary pirates, the scourge of shipping for decades, hastened the building of the U.S. Navy and began a long tradition of enforcing freedom of navigation—keeping the seas open to all commercial traffic—that has been a vital interest and core principle of U.S. foreign policy. Until now.

Two hundred and twenty-three years later, Iran’s attempted monetization of the Strait of Hormuz, implicitly endorsed by the United States in the initial memorandum of understanding (MOU) and shortly followed by U.S. President Donald Trump’s rival plan to charge a 20 percent fee on all shipping through the strait (though he withdrew the proposal the next day), may be the death of that principle. Neither the United States nor Iran have ratified the U.N. Convention on the Law of the Sea, whose Article 38 codifies commercial ships’ freedom of transit passage, while Article 44 compels states bordering straits not to hamper or suspend that passage.

The ambiguity of the MOU suggests why Iran believes that it will control the strait, which Tehran views as its ultimate source of leverage. The document’s imprecise language says that Iran will make........

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