For the Muslim view of the crusades, see the website: 'Islam and Islamic history in Arabia and the Middle East'. As the author states, 'To Arab historians, the Crusaders were a minor irritant, their invasion one more barbarian incursion, not nearly as serious a threat as the Mongols were to prove in the thirteenth and fourteenth centuries.'
The Christians of Europe had perceived Islam as a serious military threat to Christendom since the beginnings of Islam in the seventh century. The rapid military expansion of Islam under Mohammad and the caliphs had resulted in the conquest of the Middle East, North Africa, most of Spain and the western Mediterranean Islands. These areas had all been part of the Roman Empire and had been officially Christian since at least the fourth century. Muslim generals led their armies into Gaul (now France) and reached as far north as Poitiers, a few days' ride from Paris, where they were defeated by Frankish forces under Charles Martel ('the Hammer') in 732.
This was not the end of the Muslim threat, and even when the Muslims had retreated south of the Pyrennees they were still regarded as a serious danger by the Frankish rulers of southern Gaul. Not only their land forces but also their naval forces harried the Christian territories of south west and central Europe. Although driven back in Gaul, they captured Sardinia, Sicily and part of southern Italy.
By the mid-eleventh century Muslim power in Spain was beginning to disintegrate, and the Christain kings of northern Spain began slow territorial expansion southwards, in the process known as the reconquista, the 'reconquest'. In 1085 Toledo was recaptured, the ancient capital of Spain under the Christian Visigoths before the Muslim invasion. However, Muslim power in Spain was far from finished, and it was not until 1492 that the last Muslim state in Spain, Granada, was conquered. In the east Islam remained a serious threat to the Orthodox Christian Byzantine empire, based on Constantinople. In 1071 the Byzantine forces (containing many western European mercenaries) were heavily defeated by the Seljuk Turks at the battle of Manzikert and much of Asia Minor was lost to Christendom. The holy city of Constantinople, founded by the first Christian Roman emperor, Constantine the Great, seemed now to be under threat from Islam.
The loss of the holy places of Palestine to Islam in the late seventh century had been a heavy ideological blow to Christendom, although the Muslim rulers generally allowed peaceful pilgrimage to Jerusalem and the other Christian holy sites. Until the late eleventh century, Western Christendom was not sufficiently united or economically strong to conceive of any united, sustained counterattack on Islam to recover its holy sites and the territory which had belonged to the ancient Roman empire. However, by the late eleventh century, greater political stability and economic growth meant that a large expedition to assist the Byzantine emperor to recover the old Byzantine territories in the Middle East had become feasible.
Other conditions combined to bring about such an expedition: the struggle between emperor and papacy in western Europe for effective secular and spiritual control of Latin Christendom, together with the unexpected weakness of the Muslim states in the 1090s. The Muslims had long been divided into Sunnites and Shi'ites, and the Middle East was divided between the two: the Shi'ite Fatimids in Egypt, and the Sunnite Seljuks in Syria. But in the 1090s a series of deaths of leading political figures in both the Fatimid and Seljuk empires resulted in power-struggles in both empires, leaving these Muslim powers internally divided and less able to resist external attack. The Byzantine emperor seized the opportunity to ask for western European aid to recover his Middle Eastern territories. Pope Urban II took up his request for aid. He probably wanted to promote better relations between eastern and western Christendom, and possibly hoped to present the papacy as the true leader of western Christendom.
In short, the result was the First Crusade.