It quickly spread to North Africa and seeped as far west as Senegal, through a network of zawias or religious lodges. The first zawia in Libya was founded at Baida in 1844. In 1856, the Grand Sanusi founded one at Jaghbub. In time it grew into Africa's second greatest university, after Cairo's Al Azhar. The Sanusis derived strength, respect and affluence from their role mediating tribal and trade disputes in the Sahara in the days of the desert slave trade, and for providing education for the unschooled masses. 

The sun rises, blazes overhead. The road runs across the desert like a pasted ribbon, blurring off in the distance into a pool of steaming mercury. After an hour, a black smudge drifts in and out of sight on this sun-bludgeoned plateau. The tall, triple- barbed-wire fence, a surreally disfiguring structure amid these wide horizons, was constructed in 1931 by General Rodolfo Graziani, despatched by Mussolini to bring Western civilisation to Italy's "Fourth Shore". Libyans called him Butcher Graziani. Rome preferred Pacificatore della Libia. This was, in the Italian's words, "una guerra senza quartiere". Graziani herded tribesmen into desert concentration camps behind barbed wire and machine guns, poisoned their wells, condemned men to excruciating deaths in roasting salt pans, and dropped canisters of poison gas on to desert oases. Between 40,000 and 70,000 were killed. 

Sanusi fighters led the heroic, doomed resistance to the Fascist occupation under their charismatic chief Omar al Mukhtar. He was captured in 1931 and, after a 30-minute show trial, hanged in front of 20,000 tribesmen. Today his face appears on flags, street hoardings and car stickers throughout eastern Libya, a symbol of the post-Gaddafi order. His call to arms: "We will never surrender. Victory or death." The picture of a handsome old man in profile, with white beard and white skullcap, was taken by Mukhtar's Italian captors.

  

Jaghbub is an unremarkable little cluster of concrete houses. Its heart is an extraordinary expanse of rubble laid bare beneath a pitiless sun. Shattered blocks of white stone, smashed slabs of marble, sections of date-palm trunks, ancient nails, rusting spikes of wire. This is all that remains of the great zawia, architectural jewel of the oasis, that Gaddafi razed in 1988. The local preacher, Sheikh Mohammed Sanusi, a follower rather than a family member, says it took bulldozers 11 days to destroy everything within a compound measuring 47,000 square metres. "Then they finished it off with 17 explosives." 

For Gaddafi, the Sanusi name was anathema, forever associated with the benign, if somewhat ineffectual, pro-Western monarchy of King Idris Sanusi, which he overthrew in the military coup of September 1, 1969. He had the body of the Grand Sanusi disinterred and removed to an unknown   location. The sheikh says the body was miraculously preserved.

The interview with Sheikh Mohammed, a trim, slightly stooped figure of 76, begins awkwardly. He reprimands Christians and Jews for their supposed scriptural inconsistency, invites me to read the Koran, convert to Islam and earn my place in paradise. Some traditions live on. When the Egyptian diplomat, explorer and writer Ahmed Hassanein Bey travelled across the Libyan desert during an epic, 2,200-mile journey by camel in 1923, he described the order as "an ascetic confraternity [...] intolerant of any intercourse with Jew, Christian or infidel".