More than 250 years ago, in this sunbaked oasis of mud-brick houses and ramparts, the ancestors of the Saudi royal family and an outcast fundamentalist preacher formed an alliance that has shaped this land ever since. In return for political supremacy, the House of Saud endorsed the doctrine of Sheikh Muhammad ibn Abdul-Wahhab and followed it to wage jihad against anyone who rejected their creed, gaining control of much of the Arabian Peninsula. That alliance laid the foundations of the modern Saudi state, which has in more recent times used its oil wealth to make the cleric’s rigid doctrine — widely known as Wahhabism — a major force in the Muslim world. […]