The First Crusade was preached by Pope Urban II in the 11th Century. His motivation was simple-redirect potential anti-papal forces towards the ultimate reconquest, the holy city of Jerusalem.
Contemporary chroniclers and later historians have ascribed many different motives to the crusaders; the promise of a plenary indulgence guaranteeing salvation to crusaders, a disgruntled knightly class seeking an outlet for their martial energies, a grab for power to counter the Moors. The success of the First Crusade electrified Christian Europe.
The BBC documentary The Crusades provides a useful overview of the First Crusade’s impact on Europe and the Muslim world. When, after the 9/11 attack on the World Trade Center, President Bush called for a new crusade, the Moslem world had an historical frame of reference that promised, in turn, a new jihad.
The 45 minute documentary is a bit quirky but it’s worth wading through the YouTube ads. https://youtu.be/vOyswuA8wEs
Another presentation on the Crusades is a breezy overview in 25 minutes from Protestant historian Robert Godfrey, His various biases shine through but if you wanted to brush up on this crucial phenomenon just click here. https://youtu.be/kQf8sAEoZq8