Barely three weeks into a memorandum of understanding (MoU) meant to end a war, the United States is once again blockading Iranian ports, the two states are trading strikes on bases and military assets across the Gulf, and Mr Trump has floated the idea of charging shipping a 20 percent levy, or “protection money” in less polite language, for the security of the Strait of Hormuz. I would argue, however, that this is not so much the collapse of the June deal, as many are saying, as its logical conclusion. The clause on which global oil traffic depends was written to be signed, not to be operated, and the water is now exposing the difference.

Article 5 of the MoU promises “safe passage of commercial vessels” through the strait. It is an elegant phrase. It is also, as a matter of maritime law, almost empty, and that emptiness is less an accident of drafting than the price of agreement. Iran and Washington could sign because the words commit each side to very little and permit each to believe very different things. That is precisely why the arrangement is unravelling faster than even the pessimists expected.

Consider what “safe passage” leaves unanswered. It does not say who administers it. Article 5 tasks Iran with making “arrangements using its best efforts”, then sends Tehran off to hold a “dialogue” with Oman and “discussions” with the other Gulf states to define “the future administration and maritime services” of the strait. The US, the world’s pre-eminent naval power and the historical guarantor of........

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