The First Crusade was a war launched in 1099, by Christians under the support of the Roman Catholic Church, to regain control of Jerusalem from the Muslims, and to help the Byzantine Empire fight the Seljuk Turks.
As early as 1074, when Asia Minor passed into the hands of the Seljuk Turks, Pope Gregory VII had projected a war against the Muslims, which he hoped would also lead to reunion with the Greek Church. However, the plan was thrust into the background by the conflict with the emperor Henry IV over investiture and other matters.
Pope Urban II (1088-1099), who next took up the idea, was motivated not so much by the political considerations of Gregory as by actual religious impulse. The disunity in Europe in 1095 presented an opportunity for the Papacy. The calling of the Crusade was a political opportunity for Urban's pontificate, attracting loyalty from a wide range of disparate countries and asserting the Church’s role as a unifying force. From the Church came the impelling force; on the secular powers rested the actual execution of the plan. The papacy was concerned by Muslim raids on Sicily (Sicily itself was nominally Muslim). The popes were unwilling to ask the German emperor for help (as the emperors were traditionally the enemy of the Papacy), so asked the Normans to intervene. Robert Guiscard then took control of Sicily, the Pope 'granting' it to him, and it became a tightly-controlled monarchy under the Normans.
At the same time, the Spanish were continuously fighting the Moors in the Iberian peninsula. Therefore, the conception of a crusade against the Muslim world was no absolute novelty to the nations of the West.
The Byzantine emperor Alexius I was quite aware of this when he turned to Urban for aid against the Turks at Piacenza in 1095. His request met with a favourable response from the Church, as well as from the noble knights of Western Europe with their lust for adventure and conquest. When the Greek ambassadors arrived Urban was preparing for the Council of Clermont; there the Pope first preached the crusade on November 26, 1095 -- in words which have have been lost to history -- but which apparently stirred the crowd to a frenzied enthusiasm.
Urban had domestic reasons for supporting a crusade; he was French, from Rheims, and needed to give an impression of the church militant, extending the religious mission from monks to the nobility. The aristocracy supported the Crusades: many younger sons could not inherit land, and Jerusalem was an opportunity. The Crusade was available as a religious penance as much as it was an economic opportunity. Merchants and financiers from the northern Italian towns (Genoa, Pisa, Venice) also saw opportunities in setting up trading routes and outposts in the newly conquered territories.
The number of those who rose to join the Crusade increased daily, and the movement, soon exceeding papal restraint, seized upon the hunger of the lower classes. Peasants exchanged plows for arms and were joined by the dissatisfied, the oppressed, and the outcast, including members of the lower clergy, runaway monks, women, and children. This popular mob believed themselves to be led directly by God.
These events led to the legend that Peter the Hermit of Amiens, not Urban, was the true representative of the crusading idea. Peter was one of the leaders of the fanatical bands, whose contribution to the enterprise was a story of an alleged personal appearance of Jesus. According to Peter, Jesus had given him a letter describing the sad condition of the Holy Land, and commanding Peter to lead an army to re-establish Christian power there. Conveniently, most of Peter's followers were at best semi-literate. Peter also had the support of the knight Walter the Penniless, who, as his name suggests, was an impoverished knight with no lord and no vassals. Their unarmed, unorganized army had little idea of the world outside their own lands, and at every city of any great size they believed they had arrived, at last, at Jerusalem.
Their march was filled with wild excesses. The Jews were their principal targets, and many communities along the Rhine were slaughtered with the help of another peasant army led by Emich of Leiningen. On their way down the Danube, Peter's mob attacked Hungarians, Slavs, and anyone else they suspected of being "heathens." Most of Peter's army was massacred before they even reached Constantinople. Peter survived, however, and would later join the main Crusader army.
The real armies set out in 1096. The main contingents were men of Lorraine under the brothers Godfrey of Bouillon, Eustace and Baldwin of Boulogne; Flemings under Count Robert II of Flanders; northern French under Robert of Normandy (older brother of King William II of England), Stephen of Blois, and Hugh of Vermandois (younger brother of King Philip I of France); Provencals under Raymond of Toulouse; and Normans of Italy under Bohemund of Taranto and Tancred of Hauteville.
There was some dissension among the leaders, especially over who was the actual leader, though Urban himself appointed his legate Adhemar of Le Puy overall leader. The army also had to contend against the wishes of Alexius I, who was understandably suspicious of a massive army that included many of his old Norman enemies. Alexius would not let them leave until the various leaders had sworn fealty to him, and had them promise to return to the Byzantine Empire any land they recovered from the Seljuks; these oaths would quickly be broken once the Crusaders crossed into Asia Minor.
Nicaea, capital of the Seljuk "Sultan of Rum" Kilij Arslan I,was taken in early 1097, and Kilij Arslan himself was defeated at Dorylaeum. The Crusaders then marched across Asia Minor. At this point Baldwin of Boulogne set off on his own towards the Armenian lands around the Euphrates. In Edessa he was adopted as heir by King Thoros, a Greek Orthodox ruler who was disliked by his Armenian subjects. Thoros was soon assassinated and Baldwin became the new ruler; the city became the County of Edessa, the first of the Crusader states.
The main Crusader army, meanwhile, marched on to Antioch, which was captured after a long siege on June 3, 1098, but only by deception - a former Christian guard in the city opened one of the gates for the Crusaders. Almost immediately, an army from Mosul arrived to besiege the newly conquered city; on June 28 Antioch was successfully defended against this army thanks largely to the efforts of Bohemond, who claimed the city for himself as Prince of Antioch. According to legend, an army of Christian saints, including the martyrs who had been killed at Nicaea and Dorylaeum, helped rout the Turks outside the city, allowing for the success of the siege. The Crusaders also believed they were aided by the discovery of the Holy Lance inside the city.
After a break, the rest of the Crusader army marched on to Jerusalem, which had, in the meanwhile, been recaptured by the Fatimids of Egypt. After a lengthy siege in which the Crusaders probably suffered more than the citizens of the city (with 15,000 marching in starvation on July 8), Jerusalem was taken on July 15, 1099. The Crusaders massacred the whole Muslim and Jewish population, men, women and children. The Jews were burned alive in their main synagogue where they had fled; the Muslims were slaughtered in the Al-Aqsa Mosque, and according to the accounts their blood ran ankle-deep. In the days following the massacre, Godfrey of Bouillon was made Advocatus Sancti Sepulchri (Protector of the Holy Sepulchre), refusing to be named king in the city where Christ had died. In the last action of the Crusade, he led an army which defeated an invading Fatimid army at Ascalon. Godfrey died in July, 1100, and was succeeded by his brother, Baldwin of Edessa, who took the title of "King of Jerusalem". Baldwin and his successors, Baldwin II (d. 1131), and Fulk (d. 1143), extended the boundaries of the Kingdom of Jerusalem through successful warfare.
The new kingdom drew strength from the influx of new crusading forces in 1101, from the presence of the Italian merchants who established themselves in the Syrian ports, and from the religious and military orders of the Knights Templars and the Knights of St. John which were created during Baldwin I's reign.
The First Crusade marks the emergence of a self-confident, aggressive and expansionist Latin society as stability in the West left a warrior aristocracy in search of new conquests and patrimony. The new prosperity of major towns also meant that money was available to equip expeditions. The seaborne towns, in particular Venice and Genoa, were interested in extending trade. The Pope saw the Crusades as a way to assert Church influence as a unifying force, with war as a religious mission. This was a new attitude to religion: it brought religious discipline, previously applicable to monks, to soldiery—-the new concept of a religious warrior and the chivalric ethos.