From Patheos … an interview with Rodney Stark about his book on the crusades (God’s Battalions: The Case for the Crusades
)…

The Crusades did not arise ex nihilo, but were part of a broader historical and geographical narrative. Can you tell us about that?

The fact is that Islam had been attacking the west for more than 400 years before the Crusades began. Shortly after the death of Muhammad, the armies started marching. They took the Middle East, which was a Christian area beforehand. They took the Holy Land. They took all of North Africa, which had been mostly Christian. They went across the straits and took most of Spain. They took southern Italy. They took Sicily. A Muslim army marched up within 150 miles of Paris before they were turned around and run back out.

The point is that an aggressive, invasive warfare had been going on between Europeans and Islam for hundreds of years. Shortly before the First Crusade, the Normans drove the Muslims out of southern Italy and Sicily. But the Muslims were still in Spain. As a matter of fact, about thirty years before the First Crusade, the pope tried to get a Crusade going to Spain, which was about half-reclaimed at the time. And that presents an interesting contrast. Spain was close; you didn’t have to march 2500 miles; and there were riches to be had in Spain. Yet nobody went. The reason nobody went, and then thirty years later they all went, if you will, is because nobody believed that Jesus had walked around in Spain.

It really was a religious Crusade. If you don’t believe it, all you have to do is look at the mortgages, documented by Riley-Smith, where the people taking out the mortgages go on at length about why they’re borrowing the money in order to go. It was about Jesus.

Cultures change. What is overlooked about the Crusaders, and the knights and nobility of the 10th century and thereabouts, is that they were very bloody-minded. They had been raised since infancy to devote themselves to fighting. They were very sinful. They particularly were into coveting wives. And they were very religious.

The fact that these things can be combined strikes the modern mind as bizarre. But you have to deal with it if you’re going to understand these people. They would commit a horrid crime, and their confessor would say, “I don’t know if you can ever get over that one. I don’t know if atonement is in the books for you. But you better walk barefoot to the Holy Land and hope that that works.”

And they’d go. And then come back and sin some more.

It takes some stretching to understand what motivated these people.