On this day 914 years ago, 27 November 1095, at the Council of Clermont, Pope Urban II called for a holy war against the Muslims, who had been in control of Jesrusalem and much of area considered the Holy Land by Christians, Moslins and Jews alike, since the seventh century, and had recently been at war with the Byzantine Empire in Asia Minor. More specifically, it was the Seldjuk Turks who were warring with the Byzantines in Asia Minor. Although the Christians of western Europe tended to view the Islamic world as one entity, in fact different states struggled with one another and rose and fell much as they did in the Christian West. The immediate impulse for Urban's call for holy war had been a request for military aid from emissaries of the Byzantine Emperor Alexius who spoke at an earlier Council, at Piacenza in March 1095. The war was going well for the Byzantines, there was little reason to doubt that the declining Seldjuk power would soon be beaten back from the environs of the Byzantine capitol at Constantinople, but Alexius felt it would go better still with more troops. Much of the Byzantine army army already consisted of foreign mercenaries. Besides the Seldjuks in Turkey, the Byzantine army had to check the advances of tribal people across the Danube, and of the Empire's always-restive Bulgarian subjects.
However, the Byzantines felt that their appeal for military aid might carry more weight if, instead of describing the Byzantine military situation in great detail, they couched it more in terms of a struggle of all Christendom, Catholic West and Byzantine, Orthodox East, against their common Muslim foe. The Emperor's emissaries were well-received in Piacenza, and Pope Urban took their message and expanded upon it in his famous address at Clermont, held outdoors because the crowds which came were too big for the town's cathedral to hold. Urban described the infidels' control of the Holy Lands as an outrage, mentioned -- and very probably greatly exaggerated -- the hardships of Christian pilgrims at the hands of Moslims and the desecrations of holy Christian sites.
Alexius definitely got more than he had asked for, and Urban, too, soon saw the movement he had called to life grow beyond his control. It is reported that as he spoke, a cry of "Deus lo volt!" ("God wills it!) spread through the huge crowd. Before the main army of the First Crusade, made up of nobles and their followers, got underway, a more spontaneous crowd of tens of thousands, few of them skilled soldiers, mostly peasants, including many women and children, led by charismatic monks, set off from France, pausing in some cities in Germany to rob and kill Jews over the protests of bishops and other authorities. At Constantinople, perhaps at their insistence, perhaps because he was alarmed at the sight of this hungry, angry, riotous mob, numbering perhaps tens of thousands, Alexius had them promptly ferried across the Bosporus and into the path of the Seldjuk army, who promptly massacred most of them. Of those who were not killed in battle many starved or were enslaved; few ever returned to their homes in Europe.
While this fiasco reached its conclusion, the main force of the First Crusade was setting out in a rather more orderly fashion. They were hardly less alarming to the Byzantines, however, than had been the earlier mob of peasants. Alexius had asked for a few mercenaries to fill out the ranks of his army, and instead whole armies arrived, independent units whose leaders clearly had no inclination to subordinate themselves to the normal Byzantine chain of command. Indeed, to many of the western Crusaders the Byzantines seemed scarcely less foreign than any non-Christian infidels. Alexius did his best to extract oaths of fealty from the leaders of these huge armies of knights, Bohemond of Taranto, his nephew Tancred, Godfrey of Bouilion, Raymond of Toulouse and others; but there was great distrust on all sides, and later, as these western armies, like the peasants' army, were sent by Alexius as quickly as possible out of his capitol and into the fighting against the Turks and other Moslims, there were accusations on all sides of treachery and broken promises.
In the first flush of their exuberant rush toward Jerusalem the Crusaders quickly won many victories, and set up principalities for themselves, one with its capitol in Antoch, another based in Edessa, and in July 1099 they took the city of Jerusalem, and in the aftermath of their victory, in a frenzy they massacred many inhabitants of the city, Moslims, Jews and Eastern Christians, men, woman and children. It's very hard to know the number of victims of this massacre -- people tended to be much less exact with numbers in the Middle Ages -- but some Western Christian historians of the time were horrified, along with the others of the time who wrote of it; they, too, wrote of blood running deep in the streets, of thousands of helpless victims.
So that was the First Crusade, and Moslims have tended to remember the massacre which was its climax much better than it has been remembered by Christians, and so when a Western politician uses the word "Crusade" they tend to think of things like that massacre, and of a few others perpetrated since by Westerners who have called themselves Crusaders. Warriors of the Cross, killing ruthlessly, because "God wills it."