On April 28, the United Arab Emirates informed the Organisation of Petroleum Exporting Countries (Opec) it was leaving. Three days’ notice. No call to Riyadh beforehand, apparently. The grievance about production quotas was years old: Abu Dhabi had threatened to quit in 2021. What’s changed has nothing to do with the barrel count. It is about who underwrites Abu Dhabi’s security when it acts on a decision Riyadh opposes. After Iran struck UAE infrastructure, Abu Dhabi sent only a foreign...