The Sahara and the northern fringes of the Sudan form part of a zone of deserts and steppes which extend right across the Old World. According to Ibn Khaldun, the Zaghawa were Muleththemin and probability indicates that the period at which they overran the Sudan was subsequent to the first Arab invasion of the Maghreb. They succeeded in establishing themselves as a ruling aristocracy—as Shepherd Kings—throughout nearly the whole length and breadth of the Sudan. The vast steppes of Central Asia have repeatedly sent out hordes of pastoral nomads—Scythians, Aryans, Avars, Huns, Arabs, and Turks—whose conquests have shaken the world. Similarly in the Sahara, as the hottest months approach, Bedawin and Tuareg move out of the desert, northwards into the high plateaux of the Atlas and southwards into the rolling pastures of the Sudan. Although backward in developing the arts of civilization, desert nomads have always shown themselves capable of political organization and martial achievement.