“Procession of Crusaders around Jerusalem” by Jean Victor Schnetz
To understand the Crusades, you have to study the four previous centuries of Christo-Islamic history. There are loads of Islamic sources quoting Muhammad who apparently started the Arab-Byzantine Wars which ended shortly before the First Crusade. According to Islamic tradition the prophet Muhammad sent a letter of dawah, a letter of invitation to convert to Islam, to Byzantine Emperor Heraclius I. Part of the text reads as follows.
“In the name of Allah the Beneficent, the Merciful (This letter is) from Muhammad the slave of Allah and His Apostle to Heraclius the ruler of Byzantine. Peace be upon him, who follows the right path. Furthermore I invite you to Islam, and if you become a Muslim you will be safe, and Allah will double your reward, and if you reject this invitation of Islam you will be committing a sin by misguiding your Arisiyin (peasants). (And I recite to you Allah’s Statement:)” (Sahih al-Bukhari 1:1:6)
Heraclius turned down the offer, and Muhammad declared a fatwa of jihad against the Byzantine Empire. The next few centuries would not be better. The Muslims violently conquered Christian lands such as the Levant and North Africa, expanding an empire as far west as Morocco, Spain and Portugal, to as east as India, Pakistan and Afghanistan, and as north as France to as south as Yemen. This was done only in a century. Consequently many Byzantine lands were taken away, churches and monasteries were destroyed or turned into mosques, non-Muslims — especially women and children — sold into sex slavery, and excessive laws established called the Pact of Umar which prohibited Christians from being allowed to build new churches and monasteries, rebuild old ones, sound bells loud enough to be heard from the other side of the block, churches from being taller than the local mosques, images such as crosses, statues and icons could not be displayed, processions were forbidden, their literature could not be distributed, they could not convert Muslims or prevent Christians from converting to Islam, along with other laws. The only part the political correctness crowd mentions is that they had protection, but that was only if they paid the jizyah tax, and only accompanied by these excessive rules. The myth that the Muslims were tolerant was just that… a myth; the reality was different. Saint Sophronius of Jerusalem, a eye witness to the Islamic jihad, said, “At once that of the Philistines, so now the army of the godless Saracens has captured the divine Bethlehem and bars our passage there, threatening slaughter and destruction if we leave this holy city and dare to approach our beloved and sacred Bethlehem” (Christmas Sermon, 507 [p. 70]) .
“The Taking of Jerusalem by the Crusaders, July 15th, 1099” by Emile Signol
Thomas Madden, professor and chair of the Department of History at Saint Louis University, wrote that according to “Muslims the crusades . . . were simply and futile attempts to halt the inevitable expansion of Islam” . They were far from an attempt to convert people by the sword: so, for Christians they were “a war against Muslims for the defense of the Christian faith” . Madden notes, “Unlike Islam, Christianity had no well-defined concept of holy war before the Middle Ages” . The just war doctrine is different; that is meant to defend society from a violent aggressor. Jihad on the other hand is religious warfare which can either be defensive or offensive as we can see from Muhammad: “I have been commanded to fight against people so long as they do not declare that there is no god but Allah” (Sahih Muslim 1:30.33). Madden wrote that Blessed Urban II “blended the Just War theory with pilgrimage and charity” . Blessed Urban wrote the following .
“Your brotherhood, we believe, has long since learned from many accounts that a barbaric fury has deplorably afflicted an laid waste the churches of God in the regions of the Orient. More than this, blasphemous to say, it has even grasped in intolerabe servitude its churches and the Holy City of Christ, glorified by His passion and resurrection. Grieving with pious concern at this calamity, we visited the regions of Gaul and devoted ourselves largely to urging the princes of the land and their subjects to free the churches of the East. We solemnly enjoined upon them at the council of Auvergne (the accomplishment of) such an undertaking, as a preparation for the remission of all their sins. And we have constituted our most beloved son, Adhemar, Bishop of Puy, leader of this expedition and undertaking in our stead, so that those who, perchance, may wish to undertake this journey should comply With his commands, as if they were our own, and submit fully to his loosings or bindings, as far as shall seem to belong to such an office. If, moreover, there are any of your people whom God has inspired to this vow, let them know that he (Adhemar) will set out with the aid of God on the day of the Assumption of the Blessed Mary, and that they can then attach themselves to his following.”
In other words, Urban II called for “a war of liberation”  as Madden put it. It was not “a holy war”, but “an errand of mercy and an act of charity” . The Crusades also were not an anti-Semitic tirade and anti-Jewish attacks were isolated event; Madden notes that “Jews were abundant in the thriving cities along the Rhine”, and because they were under the protection of “the German crown and local lords” , some peasants began to grow jealous of the Jews’ legal status as wealthy physicians, bankers, merchants and such, so the Peasants’ Crusade, an unofficial crusade, included anti-Jewish attacks led by Count Emicho of Leiningen. In fact many “local bishops did their best to protect the Jews” . The attacks were not met without anger. Bishops and the pope threatened crusaders with excommunication if they dared attack the Jews. The Children’s Crusade on the other hand was neither a crusade nor “an army of children” as Madden mentions . It was a group of teens, women and men who were motivated by a number of things such as popular piety and even anti-clericalism as he adds ; it was not called for by the pope nor was it slavery which is the common myth. I usually don’t use YouTube videos for sources — since anybody can upload a YouTube video and make a so-called documentary — but this one is great and uses contemporary sources of the Crusades. He has many other great videos on the Crusades.
Quite honestly, though the Crusades ultimately failed to maintain the Holy Land under Christian rule, they did push back Muslims from conquering Europe. Another similar series of wars was the Reconquista. If it were not for the Crusades and the Reconquista, we’d probably be an Islamic society and women wouldn’t be allowed to show their faces, arms and legs in public in addition to not being allowed to walk in public without being supervised by a male relative. They’d also not be permitted to have a lawyer since Sharia law doesn’t acknowledge the use of lawyers. Even the LGBT community owes its thanks to the Crusades and the Reconquista because from its foundation in the 7th century to even now, homosexuals have been executed for sodomy preferably by rajm or stoning.
 Schaffen, Robert W. The Penitents’ Treasury: Indulgences in Latin Christendom. <http://facingislam.blogspot.com/2014/02/st-sophronius-patriarch-of-jerusalem.html>.
 Madden, Thomas F. The New Concise History of the Crusades. Lanham: Rowman & Littlefield Publishers, Inc. 2006. p. x.
 Ibid., p. 3.
 Ibid., p. 9.
 Letter of Instruction to the Crusaders, December 1095. <http://legacy.fordham.edu/halsall/source/urban2-5vers.html>.
 Madden, Thomas F. The New Concise History of the Crusades. Lanham: Rowman & Littlefield Publishers, Inc. 2006. p. 8.
 Ibid., p. 8.
 Ibid., p. 18.
 Ibid., p. 136.