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 navigational freedom, is not a party to those talks. Nor is the arrangement conditioned on the parties reaching any agreement. Iran is therefore free to argue that, once it has talked, it may unilaterally announce a new regime: Prior notification, designated Iranian corridors and charges dressed up as fees for “security, safety and environmental services”. Reports since the signing put the revenue Tehran imagines at tens of billions of dollars a year. An arrangement meant to reopen the strait has instead handed one coastal state a colourable claim to metre it.

The legal problem beneath the drafting is older than the MoU. The Strait of Hormuz is an international strait, and under the transit-passage regime, codified in the 1982 UN Convention on the Law of the Sea – and, in the prevailing view, reflecting customary law binding on all states – ships and aircraft of every nation enjoy continuous and expeditious passage that the bordering states may not suspend, even for security reasons. Iran, which never ratified the convention, insists that the applicable regime is instead the narrower “innocent passage” regime and that, as a persistent objector, it is not bound by the broader rule. On that reading, it may bar overflight, force submarines to surface and channel traffic as it likes. The MoU does not resolve this decades-old dispute. It gestures at “applicable international law and the sovereign rights of coastal states”. It is thus no more than a formula that each capital reads as vindication of its own position. Constructive ambiguity has become an agreement to keep disagreeing about the one point on which everything depends.

Even Iran’s narrower theory does not get it where it wants to go. Charges for mere passage are prohibited, whether the regime is one of transit or innocent passage. A coastal state may charge only for specific services actually rendered to a particular vessel, such as pilotage or towage. This is the rule that runs from the International Law Commission’s 1956 work through the 1958 Territorial Sea Convention, which Iran signed, to the modern convention. Ships have never needed special services to cross Hormuz’s wide, deep channel. Rebranding a toll as a “maritime-services fee” cannot change its legal character. Nor is the strait Iran’s to administer alone. Its southern side lies in Omani waters, and Oman is fully bound to permit transit passage there. Indeed, in the last few days, Oman has asserted in no uncertain terms that it is abiding by international law. Any Iranian interference on that side would be a plain breach of Oman’s sovereignty. Moreover, no two states, or even all the littoral states together, can bargain away the passage rights of third states that never consented, a principle as old as the law of treaties itself. The MoU cannot lawfully deliver what its most expansive readers hope it will deliver.

What the US may lawfully do in response is correspondingly constrained. Freedom of navigation permits warships to transit and to protect commercial shipping. The MoU cannot be used to license the reciprocal temptation now voiced in Washington: A proposed 20 percent US “toll” to recoup the cost of keeping the strait open.

This is where the legal fragility meets the military reality, and explains why the framework was doomed to strain. Safe passage as a legal concept presupposes a baseline of order. There should be an accepted authority, agreed rules of the road and a shared expectation that ships will not be shot at. The strait today offers none of that. Within the very 60-day “no charge” window the MoU created, Iranian forces struck vessels, including one in Omani waters; US Central Command answered with strikes on dozens of coastal radar, missile and fast-boat targets; Iran declared that it alone manages the strait and briefly proclaimed it closed; and both sides traded blows around Gulf bases. A deterrence contest is now the true governing regime of Hormuz, and deterrence is the opposite of law. It substitutes credible threat for settled rule and treats every transit as a test of resolve rather than an exercise of legal right.

No two-word phrase can carry that political burden. The MoU was made to stop a war, but now the ceasefire clause is being asked to serve as a permanent constitution for one of the world’s most important chokepoints. It is being asked to allocate power, price access and determine rights of use. Its provisions are being imposed on third parties that never signed the MoU. It cannot, should not and must not.

The views expressed in this article are the author’s own and do not necessarily reflect Al Jazeera’s editorial policy.