Even in a region beset by turmoil and learning to expect the unexpected, the lightning advance of Yemen’s Houthis along the Red Sea coast has shocked observers. More than 100,000 people have been forced to flee in a country where immiseration and displacement have become routine and half the population depend on humanitarian aid. But outsiders are paying close attention, because by claiming thousands of kilometres in a matter of days, and seizing strategically important islands, the Iran-backed rebels now control the Bab al-Mandab strait – the other major regional energy chokepoint after the Hormuz strait.

The Iran war has forced Saudi Arabia to rely on moving oil via its east-west pipeline to Red Sea ports. Now drone strikes – blamed on an Iran-backed militia in Iraq – have shut the pipeline: on one estimate, a month-long closure could cut global crude........

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