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 First Crusade began in 1095 after the Byzantines - threatened by Seljuk power- appealed to Pope Urban II for military aid. Pope Urban, hoping to divert the Christian kings and princes from their struggles with each other, and perhaps also seeing an opportunity to reunite the Eastern and Western churches, called for a "Truce of God" among the rulers of Europe and urged them to take the Holy Land from the Muslims.

Photo: The most impossing of the many fortresses built by the crusaders, the elegant krak des Chevailers in Syria held out against the Muslims for over a century and a half.

Considered dispassionately, the venture was impossible. The volunteers - a mixed assemblage of kings, nobles, mercenaries, and adventurers - had to cross thousands of miles of unfamiliar and hostile country and conquer lands of whose strength they had no conception. Yet so great was their fervor that in 1099 they took Jerusalem, establishing along the way principalities in Antioch, Edessa, and Tripoli. Although unable to fend off the Crusaders at first - even offering the Crusaders access to Jerusalem if they would come as pilgrims rather than invaders - the Muslims eventually began to mount effective counterattacks. They recaptured Aleppo and besieged Edessa, thus bringing on the unsuccessful Second Crusade.

In the meantime the Crusaders - or Franks, the Arabs called them - had extended their reach to the borders of Egypt, where the Fatimids had fallen after two hundred years. There they faced a young man called Salah al-Din (Saladin) who had founded still another new dynasty, the Ayyubids, and who was destined to blunt the thrust of the Crusaders' attack. In 1187 Saladin counterattacked, eventually recapturing Jerusalem. The Europeans mounted a series of further crusading expeditions against the Muslims over the next hundred years or so, but the Crusaders never again recovered the initiative. Confined to the coast, they ruled small areas until their final defeat at the hands of the Egyptian Mamluks at the end of the thirteenth century.

Photo: The Crusader castle at Sidon in Lebanon was abandoned after the final defeat of the Crusader Kingdom of Jerusalem.

Although the Crusades achieved no lasting results in terms of military conquest, they were important in the development of trade, and their long-range effects on Western society - on everything from feudalism to fashion - are inestimable. Ironically, they also put an end to the centuries-old rivalry between the Arabs and Byzantines. By occupying Constantinople, the capital of their Christian allies, in the Fourth Crusade, the Crusaders achieved what the Arabs had been trying to do from the early days of Islam. Although the Byzantine Empire continued until 1453, when Constantinople fell to the Ottoman Turks, it never recovered its former power after the Fourth Crusade, and subsisted only in the half-light of history during its remaining years.

For the West, however, the Crusaders' greatest achievement was the opening of the eastern Mediterranean to European shipping. The Venetians and Genoese established trading colonies in Egypt, and luxury goods of the East found their way to European markets. In the history of the Middle Ages, this was far more important than ephemeral conquests. Control of the Eastern trade became a constantly recurring theme in later relations between the European countries and the East, and in the nineteenth century was to lead to widespread Western intervention.