The Senussi were not, by origin, a warlike people. Founded in the nineteenth century, the order was a network of Sufi lodges spread across the Sahara, described before 1906 as a relatively peaceful religious community opposed to fanaticism, more concerned with teaching and trade than conquest. What changed them was invasion. When Italy seized the Libyan coast in the Italo-Turkish War of 1911, the Senussi resisted from the interior, even as they kept generally friendly relations with the British next door in Egypt. Then the larger war arrived. The Ottoman Empire, which had ruled Libya, sent envoys, among them Nuri Bey, half-brother of the Ottoman war minister, and a Baghdadi officer named Jaafar Pasha, to persuade the order's leader to turn against Britain as well. In the summer of 1915 the Grand Senussi, Ahmed Sharif, was prevailed upon to declare a jihad and strike east into Egypt, in the hope of pulling British troops away from the Suez Canal.

It was hard country to fight over. The western edge of Egypt had never even been formally drawn, the negotiations interrupted by war and never resumed. A thin coastal strip along the Mediterranean could just support grazing and the occasional well, though the wells were far apart and could run dry without warning; in summer the ground was choking dust, in the December-to-March rains it turned to glue. South of that lay bare limestone plateau, and south of that the true desert, dunes running for hundreds of kilometers. Scattered through it were oases, some large enough to hold real populations, linked by camel tracks that the Bedouin had used for centuries. Siwa, on the edge of the sand sea, was a Senussi stronghold. Whoever wanted to control this frontier had to control water, distance, and the people who knew how to move between them.

When the Senussi crossed into Egypt in November 1915, the British garrison had been stripped thin by Gallipoli and Mesopotamia, and the first months were uncertain. The defenders fell back, regrouped at Mersa Matruh, and fought a series of hard, confused actions in the coastal scrub, the Wadi Senab, the Wadi Majid, Halazin, where determined Senussi attacks were repulsed but the bulk of their forces slipped away across ground the British cavalry could not follow. The turning point came at Agagia in February 1916, where a column caught the Senussi in the open. The British captured Jaafar Pasha himself, and a charge by the Dorset Yeomanry, at fearful cost to its own horses and riders, broke the retreating column. More than that, it was technology that decided the campaign. The British brought armored cars and light Ford patrols that could cross the hard desert at speed, and aircraft that could see over it. Patrols that once reached tens of miles by camel now reached hundreds by engine. The Senussi, brave and mobile as they were, had no answer to it, and were steadily cut off from the Nile and isolated in the oases.

By the spring of 1917 it was over. The Senussi had been forced back across the border into Libya, the oases retaken one by one, the desert left to British aircraft and armored cars. The gamble had failed on its own terms: most Egyptians never rose, and the pressure on Suez never materialized. The defeat broke the standing of Ahmed Sharif, and power within the order passed to his nephew, Sayyid Mohammed Idris, who had quietly opposed the whole campaign. On 16 April 1917 the British and the Senussi signed a peace, the modus vivendi of Acroma, which recognized Idris as the leader of Cyrenaica. It was a fateful settlement. The young man who had counseled against war, and who inherited the order because of a war he never wanted, would go on, decades later, to become King Idris I, the first and only king of independent Libya. The empty frontier where camel raiders once fought biplanes had, in its quiet way, helped decide who would rule a nation not yet born.