“The Crusades” ground on for over three hundred years. Originally, the then-sitting Emperor of the Byzantine, or Eastern Roman Empire had started it all by appealing to the then-Pope, Urban, for assistance in repelling Turks who were invading the Eastern Empire. Then, it morphed into an effort to recapture the Holy Land from Arabs who had taken it during the great Islamic outburst of the late 600s. Altogether, the Popes saw them as an excellent tool for getting marauding knights to go somewhere else to practice their chosen profession, which was, basically, murder and mayhem.
The very first Crusade actually involved very few of the nobility, though. While the various kings and dukes and knights were getting organized, a popular peasant preacher and rabble-rouser raised a throng of several hundred thousand people, some few nobles, but mostly peasants, and took off for the Holy Land in the Spring of 1096. On the way, they foraged across the countryside, often killing and robbing as many Christians as they came across. They also slaughtered thousands of Jews. In any case, they got to Constantinople, and the Greeks let them through the front gates and out the back, where they soon found an army of Seljuk Turks waiting for them. Few of the peasants were even armed, and none were organized into an effective fighting force. They were massacred, and those who lived were enslaved. By the end of 1096, the Peasant’s Crusade was history. But, it was followed shortly by the First Crusade; better armed, better trained, and better organized, and that one kicked the Arabs and Turks out and declared two Christian kingdoms in the old Holy Land.
Of course, the Muslims were way closer to home, and so kicked the Christians out, again, and the Pope called for another Crusade, and then another. Some were properly organized military efforts, but some were doomed to failure, as well. In the early 1200s, there was even a “Children’s Crusade, a mass movement of European youths who hoped to convert the Saracens to Christianity, when they got there. Very few got there, most having been captured and sold into slavery long before then. The Shepherd’s Crusade suffered much the same fate for much the same reason. The last feeble Crusading effort petered out in 1497, and hardly anybody has tried to make converts with armed force, since. Besides, at about that same time, a whole new world was opening up in the Western Hemisphere, and a good many military adventurers found the gold of Mexico and Peru more attractive than the sands of Palestine, anyway. The era of Colonialism was about to begin, and the Church was there from the beginning.