The response to the Pope’s appeal was overwhelming. It is believed that over 100,000 people, an enormous number given the population of Western Europe at this time, took part in this first crusade. First, an estimated 20,000 common people without particular organization or planning followed a self-proclaimed prophet, Peter the Hermit, thinking he would lead them to a kind of paradise on earth in Jerusalem. After plundering their way through the Balkans, they were completely destroyed by a Turkish army just beyond the Bosporus.
The real crusading army, consisting of roughly 35,000 fighting men, set out on the official First Crusade several months later. This force was led by some of the most powerful noblemen of Western Europe at the time, including the Dukes of Flanders and Normandy and Count Raymond IV of Toulouse.
While some of the leaders, notably the Count of Toulouse and Godfrey de Bouillon, were men with land and riches at home, whose motives for embarking on such a dangerous and difficult expedition were largely pious, the same cannot be said of all crusaders. Bohemond, the Prince of Otranto, and Baldwin of Boulogne, for example, proved their interests were largely venal by setting themselves up as princelings in conquered territory even before reaching Jerusalem.
As for their followers, as Richard Barber writes in The Knight and Chivalry: “The Papacy saw the crusades as a way of harnessing the concept of knighthood to spiritual ends; the knights saw them as a solution to earthly ills, with the promise of absolution and heavenly reward as well …. Furthermore, by removing the discontented knights from their homes in the West, the popes believed that they would bring peace to Europe as well as helping their fellow Christians in Palestine and Byzantium.”(Barber, Richard W., The Knight and Chivalry, p. 254.) In short, many men went on crusade for practical more than religious reasons: to avoid debts, taxes, and feudal duties, for adventure, for spoils, in hope of a better future in the Holy Land ….
The First Crusade reached Constantinople in April 1097. Here the leaders dutifully swore fealty to the Byzantine Emperor, in whose name they continued as soldiers of an Emperor seeking to re-take territory that had belonged to his predecessors. The crusaders then crossed into Muslim-held territory for the first time (modern Turkey) and confronted Seljuk forces on July 1. The crusaders routed the Seljuk army and continued east until they came to Antioch.
Here, after an eight-month siege, Antioch fell to the crusaders – who promptly found themselves under siege by a much larger Muslim army. The crusaders appealed desperately for aid from the Byzantine Emperor, their ‘overlord,’ who had sent them on this mission and promised them support. In fact, the Byzantine Emperor had moved his troops in behind the crusaders, “mopping up” what remained of the Seljuk forces and re-establishing Byzantine control over Asia Minor. But, unfortunately for Christianity, one of the crusaders, Stephan of Blois, deserted the crusader cause and on his way home told the Byzantine Emperor that the crusaders were defeated and lost. The Emperor therefore decided to consolidate what he had and returned to Constantinople, leaving the crusaders on their own.
Trapped in Antioch, on the brink of starvation, the crusaders discovered a relic, which one of the priests identified as the lance that pierced Christ’s side before the Crucifixion. This “miracle” inspired the crusaders to undertake what turned out to be a decisive sortie that drove the besiegers off. But the survivors no longer trusted the Byzantine Emporer, or felt that they owed him fealty. Instead, they established two independent kingdoms, at Edessa and Antioch before the hard-core of the crusaders pressed on for Jerusalem itself. A year later, the weary, much decimated, ill-equipped, and half-starved crusading army reached Jerusalem.
This army had suffered extreme privation during its march, notably thirst so intense that according to the chaplain of the Count of Toulouse, at one pool “those who were strong pushed and shoved their way in a deathly fashion through the pool, which was already choked with dead animals and men struggling for their lives …. Those who were weaker sprawled on the ground beside the pool with gaping mouths, their parched tongues making them speechless, while they stretched out their hands to beg water from the more fortunate ones.” (Hopkins, Andrea, Knights: The Complete Story of the Age of Chivalry, from Historical Fact to Tales of Romance and Poetry, p. 85.)
The army was no longer large enough to encircle the city, so no proper siege was possible. Furthermore, the leaders believed that a Muslim relief army from Egypt was on its way, which made a rapid victory all the more important. The crusaders therefore attempted to take the city by storm almost at once, but lacked sufficient ladders to scale the walls; the assault was driven off. An imperfect siege began, during which the Christians secured materials to build siege engines. On the night of July 13, 1099, a new assault was launched, but it was not until the afternoon of July 15 that a breakthrough was achieved.
The crusaders reportedly poured into Jerusalem. At this moment of their greatest triumph, the crusaders committed the atrocity that has besmirched the very words “crusades” and “crusaders” ever since. Knowing the Christians had been expelled before the siege, the crusaders put all the defenders to the sword. Later accounts would claim that the crusaders' horses waded up to their fetlocks in running blood, but serious historians note that in fact thousands of inhabitants survived the capture of Jerusalem, so that oft cited accounts of wading in blood were exaggerated -- not to say allegorical. All Muslims were, however, expelled turning Jerusalem into a Christian city once again.