Jerusalem was conquered by the Muslim Arabs in 637 as part of their rapid conquest of the Middle East, Persia and North Africa while in Europe Roman society had been overrun by barbarian migrations. Of course the Arabs were also just barbarians, barbarians with camels. It was the empires they conquered that had all the culture, knowledge and luxury. The fighting between the Islamic rulers and the rump of the Byzantine Empire continued for centuries. Fighting within the Islamic Empire also continued as there wasn't a lot of brotherly love in the Dar al-Islam. The Crusades were kick-started by mismanagement of the Jerusalem tourist trade when the Seljuk Turks conquered Jerusalem in the 1070s. At first they didn't understand that it was pilgrim gold providing their loot.
In 1095 the Byzantine Emperor Alexius requested help against these Seljuk Turks from Pope Urban II and for a complex range of religious, political and financial reasons the Pope called for a "Holy War" to retake the "Holy Land for Christianity." The Crusaders probably meant well and, after all, the Muslims had invaded and conquered the area themselves but it's quite possible to view the Crusades as one of the worst examples of conniving, backstabbing, stupidity, viciousness and atrocities in human history and that's just amongst the "Christians" whose dirty deeds are now well-known. The Muslims were just as bad as it was only their uncivil wars and conniving, backstabbing and internecine warfare that allowed "the Crusaders" any possibility of victory. In the end, despite some victories and some coastal Palestinian land held for 2 centuries the Crusades were a dismal failure.
Today there is a lot of resentment against the West amongst Muslims supposedly due to the Crusades but this is completely unwarranted. There are no calls for jihad against the Mongols whose 13th century depredations against Muslims makes the Crusades look like a tea party. When they conquered Baghdad up to 1,000,000 Muslims were killed and by the time the Muslim lands were recovering from the Mongol terror, the Muslim warrior Timur the Lame came out of Uzbekistan in the late 14th century and attempted to break the Mongols' world records for annihilation.
Timur's armies devastated Central Asia, Persia and Iraq, India, Egypt, the Middle East, Russia, Turkey and all points between. He was an unparalleled devastator and despoiler but there are no calls for jihad against Uzbekistan though it's estimated Timur's unwarranted and pointless wars killed 17 million people, amounting to about 5% of the world population at the time, mostly Muslim.
The reality is that Muslims have killed far more Muslims than Christians, Buddhists, Hindus and any and all other miscellaneous people added together have and in nearly all cases the Muslims were doing the invading.
- Fields of Blood - Karen Armstrong
- The First Crusade: A New History - Thomas Asbridge
- The Crusades: The Authoritative History of the War for the Holy Land - Thomas Asbridge
- Muslims and Crusaders: Christianity’s Wars in the Middle East, 1095-1382, From the Islamic Sources - Niall Christie
- The First Crusade: The Call From the East - Peter Francopan
- Saladin: Hero of Islam - Geoffrey Hindley
- In the Shadow of the Sword: The Birth of Islam and the Rise of the Global Arab Empire - Tom Holland
- In God's Path: The Arab Conquests and the Creation of an Islamic Empire - Robert G. Hoyland
- The Lost History of Christianity: The Thousand-Year Golden Age of the Church in the Middle East, Africa, and Asia - and How It Died - John Philip Jenkins
- The Crusades Through Arab Eyes - Amin Maalouf
- Holy Warriors: A Modern History of the Crusades - Jonathon Phillips
- God's War: A New History of the Crusades - Christopher Tyerman