The 1st Crusade was a military expedition by Christian knights to retake lands captured from the Eastern Roman Empire by Muslim powers from 1096 to 1101.
The declared casus belli of the First Crusade was to restore to the Eastern Roman lands that had been taken by the Arabs and Turks over the past few centuries. It was also suggested at the Crusades outset that reclaiming the Christian holy city of Jerusalem. The motivation for the Crusaders was mainly idealistic; defeating the Muslims who had previously encroached on Christian lands was their main goal.
Arguably one of the main motivations of Pope Urban II when he called the Crusade was to reunite Christianity (current split between Orthodox and Catholic churches) under his authority.
Precedents for the Crusade include the campaign against the Zirids, later recognized as a crusade. Christians motivated by religion had fought Muslims in wars across the Mediterranean since before the 11th century. The first crusade was merely the largest (at the time) example of this phenomenon.
Pope Urban II sent envoys to all nearby Christian kingdoms in early 1096 to raise support for the declared Crusade, receiving an overwhelmingly positive response. Knights from Provence, Normandy and France, organized into two hosts, one under Adhemar du Le Puy and the second under Raymond IV of Toulouse, made their way to Genoa. Following negotiations which promised one-third of any city captured to the states of the League of the Tyrrhenean in battles that the League contributed forces, the League agreed to provide transport for the Crusaders.
At the beginning of the year 1097, the Crusader forces reached Constantinople. Emperor Alexios I Kommenos welcomed the Crusaders and resupplied the christian army, in turn receiving a pledge that the Crusaders would turn over lands conquered to the Eastern Roman Empire. Alexios also augmented the Crusader army with a large Roman contingent.
In early 1097 the combined crusader forces crossed the Bosphorus and entered the territory of the Sultanate of Rum. The first battle of the Crusade occurred at Nicaea, capital of the Turkic Sultanate. Turkic forces met the Crusaders in battle at Dorylaeum and were thoroughly routed.
At this point Baldwin of Boulongue departed the main Crusader force and convinced the childless and unpopular Thoros of Edessa to make him his heir. Thoros was soon dead after a uprising, possibly instigated by Baldwin. The County of Edessa was the first vassal state the Crusade bequeathed to the Eastern Romans.
In early 1098 the Crusaders reached the city of Antioch. Antioch was a well fortified city and considered unassailable. The Crusaders were reinforced by a German army, the so called "Saxon Crusade", at this point. Much needed supplies were provided by the League merchant fleets, which helped shield the Crusaders from attrition while they fought off two large Fatimid relief armies.
In the meantime, Byzantine armies attacked Damascus and Aleppo, which both fell to the Crusaders.
An Armenian traitor was bribed into surrendering his tower, allowing the city of Antioch to finally fall. With Antioch captured, the Crusaders turned south. Upon reaching Jerusalem in mid 1099, the next major city to offer significant resistance, the Crusaders were again running low on supplies. After the first failed assault a Genoese fleet arrived, providing timber for building siege engines and skilled miners.
Hearing news of a Fatimid army marching north, the Crusaders rallied and took the city, massacring the population. The crusader army marched further south to Ascalon, where the Fatimids were routed.
At this point dissension split the Crusader camp. Some, urged on by the Byzantine contingent, wished to finish the crippled Fatimid Caliphate. Others wished to return home or to their conquered lands, seeing the Crusade as finished. The League refused to ferry deserting crusaders back home, leaving them to find an alternate method of Travel.
A League fleet carried as many crusaders as possible around the Sinai peninsula to Damietta, where the city fell after a brief siege.
The Crusaders then marched south, taking Cairo. The Fatimid Sultanate dissolved, and the exhausted and depleted crusaders were allowed to settle or return to Europe.
Without the Fatimid and Seljuks being in dramatic inner turmoil at the time, it is doubtful that the Crusade could have succeeded. As it was, it did.
Many landless knights chose to stay in their conquered territories and administer them for the Eastern Romans. The Eastern Roman Empire found itself suddenly restored to a large portion of its former territory. The conquest of Egypt was a particularly beneficial outcome of the Crusade. Unfortunately, the Roman militia system was not yet established in their newly conquered territories, and the Romans were forced to allow large swaths of their territory to be run as vassal states to defend them.
The Fatimid Caliphate completely and utterly collapsed in the Crusade, ending its existence.