Omega Vision
Face Flowed Into Her Eyes
I think it's probably true that lots of the Crusaders believed that the Crusades were a "best defense is a good offense" situation, but what they believed they were doing and what they were actually doing are two different things, and beyond that you can't point at the operations involved and say they were in the interest of defending Europe. Sacking Constantinople for instance.
I've also seen theories that the crusades were a scheme by the Papacy to ensure relative peace in Europe. After all if all the big kings and their armies were away fighting Muslims they couldn't very well start wars in Europe.
There are examples of defensive crusades though, as when the Mongols invaded Central Europe. But those are very specific examples, and were literally in response to a current invasion--nothing like the invasions of the Middle East that the Crusaders launched.