In fact, I'll just quote the entire aside from the book here.
The Fatimid dynasty had historically exercised religious tolerance; most subjects in Egypt were Coptic Christians, Jews and Sunni Muslims. But some rulers treated Christians and Jews more harshly. One, Caliph al-Hakim (r. 996-1021), ordered the destruction of Jerusalem's Church of the Holly Sepulcher, inflaming Christians. (Al-Hakim, considered to have been mad, is also alleged to have been behind an effort to destroy the sacred Black Stone in the Kaaba, perpetrated by a fanatic who was killed in the midst of his attack.) Likewise, Christian pilgrims visiting the Holy Land often encountered adversity in the Muslim lands they traveled through. These events helped create an antagonistic atmosphere between Christendom and Islam. In 1094 the Byzantine emperor Alexius Comnenus, who was losing Asian territories to the Seljuks, asked Pope Urban II's help in fighting the Islamic forces. A half century earlier the Seljuk Turks had overrun the western territory of the Abbasid dynasty, before Tugrul Bey established his sultanate in Baghdad in 1055, under which the Abbasid caliph served. The Holy Land itself,at the time of the Crusades, was a disunited region ruled by competing chiefs. The Seljuk Turks controlled the north. In 1095, the year after the Holy Roman Emperor asked for his assistance, Pope Urban II called upon the faithful of Christendom to march on the Holy Land and "wrest it from the wicked race" occupying it. The First Crusade, so named for the cross under which the men marched, left Constantinople in 1096. Some 30,000 to 35,000 soldiers formed the fighting core of the rag-tag amalgamation of perhaps 150,000 crusaders from across Europe that set off for the Holy Lands in the Levant. A series of Crusades followed, lasting until 1291.Except for pilgrimage traffic, the Crusades didn't affect Arabia too much. The Crusaders only once threatened the holy cities of Medina and Mecca, but were soundly defeated by the Arabs.
The 200 years of fighting was a series of victories and losses for both sides. One decade the Crusaders would capture Jerusalem and much of Palestine, the next decade the Muslims would take it back. In 1291 the Muslims finally drove the Franks off the coast in Antioch, Tyre and Tripoli.
It hardly mattered, though, as the Islamic Empire had been brought down by the Mongols by this point.