Iran is demanding ships pay up to 2 million dollars to pass through the Strait of Hormuz - with a simple argument: if Suez and Panama can, why not them? The question sounds legitimate, right up to the moment international maritime law gets opened. That is when a tidy distinction shows up - one that is rarely spelled out aloud in diplomatic practice: canals are man-made, straits are natural.

Tehran justifies the levies as "war reparations" and compensation for "navigational services", environmental protection and enhanced security. It is planning a joint protocol with Oman to require special permits for entering the strait. Some Asian shipping companies have quietly agreed to pay. The main global operators - the United States, China and all the Gulf states - are flatly refusing.

Why doesn't this fly? According to maritime experts, UNCLOS (the UN Convention on the Law of the Sea) guarantees "transit passage" through natural straits and prohibits the coastal states from blocking it. Only limited fees are allowed for services such as pilotage and towage. Everything else counts as a breach of international law.

Suez and Panama are a different category. They are man-made waterways, built and maintained by sovereign states at enormous expense. That is why Suez generates around 4 billion dollars a year in transit fees; Panama charges in the same way. The international regime recognises that as an investment being recouped. Hormuz, as a natural strait, is not a construction project - it is there on its own.

What Iran is doing now is testing international law with an economic lever. With one fifth of the world's oil and gas passing through Hormuz, the revenue potential is huge. But the conflict potential is bigger. When one country starts charging for what the law says should be free, the others have two options - pay, or say "no" with a gun. And precisely because of that dilemma, Hormuz has been the most unstable shipping route on the planet for three months now.