I was just debating with a catholic acquaintance. He ascribed merit to the catholic crusades and said that many historians agreed with him. Are there any non-catholic historians who believe that the crusades were not motivated as a distraction from internal conflicts? His chief comments were that the crusades protected Europe from invaders and liberated Israel. I was taught that the crusades were a distraction from problems in Rome and the pope. What does history actually say was the multifold motivation for the crusades?
I think you can't really separate the two sets of motivations for the crusades (religious fervor/ political or power-grabbing issues) from one another. In a time where politics and religion were habitually and naturally intermixed it's hard to expect something else.
A look at the least of leaders of the First Crusade to examine their personalities can be useful here (it's a study in mini-prosopography, if you like):
To sum, it was a (un)healthy mix of religious fervor, greed and striving for glory.
As for the context of the Crusades, they were sort of a counter-movement by Christian Europe against the previous great Moslem movement of conquest. For example, Raymond had fought against the Moors in Spain before the Crusade - so there was clear continuity between the Spanish Reconquista and the Crusades.
Even as someone with an Anglo-Saxon background, I find it informative to look upon the Crusades in the way the residents of the area must have viewed them - a series of barbarian invasions from the north-west.
Yeah, the Crusaders had their reasons. In their own minds they were completely justified. But the same could be said for the Huns and the Mongols and the Vikings. If we are going to give their side that much weight, we should at least be willing to do the same for other wave invaders of history.
So while I could list some "motivations", I don't really find them nearly as compelling as it appears your friend does. The main issue was that there was a percieved military imbalance. Europeans thought they could whip the Turks and the other near-east rulers if they acted together. The rest is just the rationalization for doing so that they happened to arrive at.