3 io THE HISTORY OF MEDIEVAL EUROPE arts, sciences, and industries scattered through their terri- tories; they also failed to reunite the Moslems into a politi- cal whole. Various leaders broke away from the control of sultan and caliph and conquered independent principalities for themselves. This naturally led to many wars between rival Mohammedan princes. For the present, however, the Turks were none the less a pressing danger to Constantinople, and when Alexius Alexius (1081-1118) firmly established the Comnenian Comnenus dynasty on the throne, he still found many prob- lems confronting him. Robert Guiscard, whose daughter had married the son of a preceding emperor, invaded the northern part of the Greek peninsula and penetrated as far as Thessaly. He was then called back to Italy to succor Pope Gregory VII, and in his absence his forces were ex- pelled from Greece. He continued the war until his death in 1085, however, but then his son Bohemond made peace with Alexius. Meanwhile the Patzinaks had been invading Thrace, and it was only after nine years of war that Alexius finally drove them out of his empire. He was next confronted by the far more arduous task of repelling the Seljuk Turks, but in this enterprise he was destined to receive assist- ance from vast armies of crusaders from western Europe. Our sources concerning the crusades are more ample than for any other wars or migrations of the Middle Ages. Besides Sources for numerous chronicles concerning them, there are the crusades diaries and letters written by the crusaders themselves. There is also a wealth of official documents bearing in one way or another upon the crusades and the states founded by the crusaders in the East. There were also numerous allusions to the crusades in the popular litera- ture of the time. Yet many important points are still left in dispute; for instance, whether Alexius summoned the crusaders or not. Moreover, the narrators of the crusades introduce so many portents and miracles, and are themselves so convinced that these expeditions were especially favored by divine guidance and by providential intervention at critical moments, that their accounts sometimes seem to-
