 Local tribes defected from the Mahdists, forcing Osman to retreat to the mountains.He remained a leader in the Mahdist army but was only marginally involved in the conflicts that led to the final defeat and death of the Khalifa.When the British under Herbert Kitchener moved into the Sudan in the spring of 1898, the Khalifa sent a force under Emir Mahmud Ahmad to join with Osman's army.