Were the Crusades a bad thing?

This subject came up in a discussion last night, as part of someone's anti-Catholic diatribe. We need to be clear about this. The Crusaders behaved abominably. They were, however, a necessary response to four centuries of aggression, at the request of the Byzantines who were in the front line and needed help. They ultimately failed.

Or perhaps the Crusades have never really ended. The westward spread of Islam was not checked until 1683 when the Ottomans were defeated when they besieged Vienna. The Ottomans were slowly driven back from most of the Balkans and Greece. However, the Christians of Asia Minor - the ancient communities of Armenians and Greeks, paid a terrible price at the beginning of the twentieth century, when three million died in the two genocides of 1915 and 1923, at the break-up of the Ottoman Empire and the foundation of the state of Turkey.

We should no more condemn the Crusades than we should condemn the Second World War on account of some of the acts of the British and US military during the course of that war. Hiroshima, Dresden and Hamburg, for example, do not negate the rightness of the war itself. In the case of Crusades, we are judging men who have been dead for centuries by the standards of today. That is absurd. Bad things were done and we should admit that, but we should stop apologising for the Crusades as such. If there had not been four centuries of Muslim Arab aggression there would have been no Crusades.