Which strait's closure has been the standing threat to a fifth of the world's oil?

Hormuz, between Iran and Oman, about 33 kilometres wide at the narrowest with shipping lanes only a couple of kilometres across in each direction. Roughly a fifth of global oil consumption passes through it, and there is no adequate way around.

The threat is credible because closing it is easy and reopening it is not. Mines, shore-launched missiles and small fast boats are cheap; clearing mines from a busy strait under fire is slow, and insurers stop writing cover long before the shipping physically stops. Iran has rehearsed this repeatedly without doing it, which is the point - the threat is worth more unused.

It would also hurt Iran, whose own exports go the same way, and China, its largest customer. Saudi Arabia and the UAE have built pipelines to the Red Sea and the Gulf of Oman to bypass it, but their combined capacity covers only part of the flow. The Strait of Malacca is the other chokepoint of comparable weight, carrying most of the oil bound for East Asia, and the reason a great deal of naval planning is about narrow water rather than open ocean.