Yves Lacoste over at the Verso blog:
Many contemporary historians and specialists in North African history give the impression that the major interest of Ibn Khaldun's work is that it provides us with a complete explanation of the crisis that put an end to the social and economic development of the Maghreb. They argue that the crisis was the result of the gradual invasion of North Africa by nomadic Arab tribes from the east, first the Beni Hilal and then the Beni Solayn. According to C.A. Julien, the most famous specialist in North African history, the Hilalian invasion was “the most important event of the entire medieval period in the Maghreb.”1 It was, he writes, “an invading torrent of nomadic peoples who destroyed the beginnings of Berber organization — which might very well have developed in its own way and put nothing whatever in its place.”2 It must be stressed at the outset that The Muqaddimah does not provide a systematic account of this crisis, the effects of which were still visible in the fourteenth century. Ibn Khaldun gives no methodical account of the underlying causes of this destructive phenomenon. The Histoire des Berbères describes a series of upheavals and crises, and several unsuccessful attempts to establish a centralized monarchy. But the problem of a Crisis with a capital 'C' is never raised. The Hilalian invasion is not the main theme of the The Muqaddimah. Ibn Khaldun refers to it simply as one of the causes of the turmoil.
The encyclopedic Muqaddimah contains a section on methodology, an analysis of political and social structures, and a general synthesis, but basically it does not describe the spectacular collapse which modern historians claim to have discovered. Ibn Khaldun was not studying a major localized event such as an invasion and its aftermath; he makes no systematic distinction between the character of the Maghreb before and after the crisis. But he does make a methodical analysis of the permanent political and social structures that characterized North Africa. And, according to Ibn Khaldun, the arrival of the Hilalian tribes did not alter those structures to any great extent. No space is given to a detailed study of the Hilalian invasion in the systematic and analytic framework of The Muqaddimah or in the Histoire des Berbères, each chapter of which deals with a different dynasty.
The lengthy modern accounts of the Hilalian invasion do not, therefore, derive directly from Ibn Khaldun. It is, of course, quite legitimate to formulate a thesis by collating scattered data. But the theory that the “Arab invasion” was the determining factor in the crisis of medieval North Africa is less than legitimate, as it takes into account only part of the data provided by Ibn Khaldun.