Dr. Brendan McGuire gave a wonderful free series of lectures on the Crusades over at the Institute of Catholic Culture. In one part of his lecture, Dr. McGuire examines the reasons why crusaders went to war. The secularizing, anti-Catholic historians of the 19th century focused on the second son theory and the estates theory – both of which are still emphasized in pop-history literature, movies, and television documentaries. Dr. Jonathan Riley-Smith, the most prolific living historian on the crusades, and a Knight of Malta, debunked both theories through use of a historical method called prosopography (i.e. the study of history based on the creating of databases on individuals who participated in an event).
According to the second son theory, titles and wealth went down to the firstborn son and thus the poorer knights of Europe would leave for the riches of foreign lands. In actuality, this theory is simply 17th-18th century European colonialism anachronistically projected into the medieval past. Riley-Smith, using a computer-driven database of medieval charters, discovered that the vast majority of crusaders in the First Crusade were wealthy land owners who had to expend 4-5 years’ worth of annual income up front in order to fund their part of the crusade. Now for an economy with reduced monetary system, liquid assets had to come through selling off titles or land. A surviving crusader who returned home would thus have very little to return to.
The estates theory seemed to correct the problem of the second son theory by stating that crusaders never intended to return home. They would instead carve out large, new kingdoms for themselves in the Holy Land. But this theory suffers from an even greater problem: 99% of surviving crusaders returned home. In fact, the true day-to-day defense of the Holy Land would turn more and more to the military orders of the Hospitaller Knights and the Knights Templar. But if crusaders did not fight for wealth or land, what did they fight for? They fought not merely for the personal spiritual benefits of indulgences, but also because their German blood boiled at the thought of the humiliation imposed on Christianity in the Holy Land and across formerly Christian lands. It was the same German warrior spirit, tempered and directed by faith, that forged both the nations as well as the great crusades.