In the midst of the Middle Ages hundreds of thousands of people left their homes and families and marched off to conquer Jerusalem. What could motivate so many people to march on foot for months and risk their lives in a war from home? In order to understand who these people were and what their motivation for joining such a pilgrimage might have been it is necessary to first know a bit about medieval society.
That society consisted of three castes. There was the nobility, the clergy, and the peasants. The peasants worked the land of the nobility and were, essentially, owned by the nobles who could do whatever they pleased with them. Neither the nobles nor the peasants had much education; only the clergy, barricaded in their churches and monasteries, had any access to learning and knowledge. They used this very much to their advantage as they inculcated the masses with religious fervor and superstition, thus keeping themselves an extremely powerful force in the daily life of the Middle Ages.
It was also a time of extreme filial hierarchy. Power, titles and land passed from father to son in a direct line. Each noble passed on everything he had to his oldest son. The rest of the sons were considered noblemen but did not inherit their fathers land or money. Having nothing else to do, many of them became knights. Knighthood was not the safest occupation as the knights tended to spend their time training for, and fighting in, the many wars of the period. For the most part, the knights fought each other, but being able to fight the Moslems instead was quite an attractive idea to them.
On their own, however, the knights had little power to affect such large-scale warfare. It was the clergy which was the direct cause of the Crusades. They convinced the nobles, including many kings and princes, that conquering Jerusalem was a religious duty. It was an easy sell to the highly religious and superstitious masses. Holy war appealed to many of them, for a variety of reasons.
The knights, of course, were happy to join because they were always ready to fight. War was essential to their occupation and the lure of far-away battles in the Middle East only exacerbated their bloodlust. The nobles were pleased with the chance to conquer more land and cover themselves with the glory of victory in battle. As for the peasants, they were promised freedom from any debts they owed if they joined the holy war. For an impoverished and subjugated peasant, this was a chance to finally break free of serfdom. It also afforded them the opportunity of free travel to the exotic Middle East. The kings had no trouble recruiting huge armies to march on Jerusalem.
What they did have trouble with, though, was implementing discipline on those armies. The soldiers looted, plundered and destroyed whatever they could get their hands on as they passed through Europe. Their leaders were no better; they were constantly busy with internal power struggles and intrigues. There was no unity or any real sense of purpose. It was no surprise then, that the Crusaders ultimately failed in their conquest.