What was the most significant reason for the success of the First Crusade?

On July 15th 1099 Jerusalem, recently captured by the Fatimids of the Egyptian Sultan, Al-Musta'li, fell to the crusaders. With their victory, three years of campaign came to a victorious end; the assurances of Urban II had been satisfied, and as many then did, the Franks could return to Europe. Yet how did a numerically inferior force, enfeebled by repeated siege, battle and guerrilla warfare, defeat their multitudinous foes?When the crusaders crossed into Anatolia in 1097, they were confronted by an Islamic world riven by sectarianism. The Seljuk Empire, which encompassed large parts of the Levant, had collapsed at the death of Malik-Shah - the Seljuk Sultan. As his offspring partitioned their father's dominion, powerful Seljuk warlords availed themselves of the chaos, seizing their own territories. By 1095, fiercely independent city states had arisen, prone to warring amongst themselves rather than uniting against a common foe. The fragments of the Seljuk Empire were embroiled in interminable rivalry. Moreover, the Egyptian Fatimids and the Abbasid Caliphate, the latter replaced virtually by the Seljuks, were similarly irreconcilable. The rift between the Sunni and Shia, the denominational parties of Islam, prevented cooperation between these great powers, for Fatimids swore faith to the Caliph at Cairo, the Abbasids and Seljuks, to the Caliph at Baghdad. When the crusaders besieged Antioch in 1098, the Egyptians were far more eager to negotiate with them rather than assist the Sunni in control of the city. Had the forces of Islam united against the Latin threat they would have likely prevailed, but instead, the Franks exploited their disagreements, assailing a foe at the nadir of their power. This was to be the most important reason for the eventual Christian victory.