The primary cause of the First Crusade was the Papal response to repeated pleas for assistance from the Byzantine Emperor Alexius Comnenus, whose lands had repeatedly been invaded by the Seljuk Turks (a process initiated following the crushing Byzantine defeat at Manzikert in 1071). Alexius asked for aid in repulsing the Turks and reconquering territory lost in Anatolia (frequently labeled as the "heartland" of the Byzantine Empire). It's important to note that the request for aid was specifically against the Seljuk Turks, and not the Fatamid Caliphate - the Muslim empire that held Egypt, the Levant (including Jerusalem) and Arabia. Emperor Alexius was on good terms with the Fatamids, both of whom viewed the Seljuks as a greater threat to regional stability than each other.

Pope Urban accepted the requests for aid and called for the crusade in 1095 for two reasons: the first is relatively straight-forward: he was the Pope and out of basic religious feeling wanted the Holy Land to be restored to the Christians. The second reason is that he wanted to strengthen his position in the Papal States, and also unify Christendom for a single noble cause. He wanted an end to the constant fighting amongst themselves, and thought that if he organised the crusade under the moral leadership of the Papacy, it'd reassert Papal supremacy in Christendom over encroaching secular rule.

The three major regions of Europe that responded to the crusade were France, Germany and Italy. Between 1095-6 thousands of soldiers under the command of a large variety of noblemen, both high and low, "took the cross" and pledged themselves to go on crusade. However, before these noble-led armies were ready to depart, another "army" had already left, the so-called "People's Crusade", led by a charismatic preacher and consisting almost entirely of poorly armed and poorly trained bands of peasants, who had a lot of enthusiasm and piety but no strategic skill. As they travelled across Germany they committed many atrocities on Jewish communities and in general wreaked havoc in the countryside. When they finally reached Anatolia they were promptly destroyed by a Seljuk army.

By 1097 all of the major noble-led armies were ready, and they departed. The original destination was Anatolia, not the Holy Land. The two possible ways to get to Constantinople - the agreed upon meeting point - was either by land or by sea. Going by land was cheaper but longer. Going by sea was more expensive but faster. In 1097 the armies reached Constantinople one after the other, and the leader of each army (as the crusade was not done by a single large army, but several smaller armies commanded by unrelated noblemen from different parts of Europe) was made to swear loyalty to the Emperor Alexius and promise to restore any reconquered lands to him, since that was, of course, the whole purpose of the crusade.

Throughout 1097 the crusader armies crossed Anatolia, reconquering land seized by the Seljuks. Since Byzantine armies accompanied the crusaders, they were able to garrison these places and ensure the land was returned to Alexius. The crusaders won a few victories against Seljuk armies and by the end of the year they'd reached the border into Syria. The Siege of Antioch (1097-8) was a major event; it was a prolonged siege of the vital city linking Anatolia with the Holy Land. After many months it was captured by the crusaders (who then repulsed a counter-siege by the Seljuks). The crusaders refused to hand over the city to the Byzantines, who were understandably furious and basically packed up and went home to defend their land in Anatolia.

At this point it was believed the crusade was all but over, since almost all of Anatolia had been reconquered and the crusaders had decisively defeated the Seljuks multiple times, but the proximity to the Holy Land (and thus Jerusalem) was too strong a temptation for the crusaders, and they decided to set retaking the Holy City as their next objective. They promptly set out, invading the Fatamid Caliphate and marching down the coast, seizing several important coastal cities along the way (e.g. Acre, Tyre). Emperor Alexius was furious when he found out, since as mentioned he was on friendly terms with the Fatamids and had no interest in conflict with them. He distanced himself from the invasion and even apparently refused to aid the crusaders anymore, either with supplies or transportation.

After a desperate siege where the crusaders came very close to starving and dying of dehydration because of the land around Jerusalem had been stripped bare by the defenders, the city was taken in 1099 after timber from recently arrived Genoese ships was used to build siege engines. Since the city had refused to surrender, custom dictated that the army be entitled to plunder and sack it, which they did quite ruthlessly.

The fall of Jerusalem marked the climax of the crusade, but not its end. The war continued for a few months as crusader armies marched further south through Gaza and towards Egypt, seizing more land along the coast to establish a buffer zone between the Fatamids in Egypt and Jerusalem.

The political map of the Holy Land was now vastly changed by 1099; most of Anatolia had been reconquered for the Byzantines, and all of the Levant had been lost to the Fatamids, which was now replaced by an assortment of "crusader states", most notably the County of Edessa in north-western Syria and southern Anatolia; the Principality of Antioch in western Syria; the County of Tripoli in Lebanon, and the Kingdom of Jerusalem in Palestine and Gaza. Each of these states was ruled by a European nobleman, and while most of them pledged nominal allegiance to the Kingdom of Jerusalem, this was mostly a formality and they functioned as basically independent. The Principality of Antioch was technically a vassal-state of the Byzantine Empire, as its lord had agreed with Emperor Alexius, but this relationship was stormy and unreliable. For their assistance in the transportation of troops and the sieges of the coastal cities, the Italian maritime republics of Venice, Genoa and Pisa were given their own quarters in some of the cities as reward, so they were some of the other lesser known victors in the crusade.

Source: God's War: A New History of the Crusades by Christopher Tyreman