The history of the crusades is a bit complicated and there are a number of factors that lead to the First Crusade, one of them being an appeal from the Eastern empire in Constantinople to help repulse the advancing Isla-mic Seljuk Turks.
The Mus-lims had wanted to take the city from the 8th century but could not and they kept at it until 1453 when they finally took it. It is interesting that when Mehmet took it he used Christian gun expertise from the Balkans and Christian mercenaries or those forced into his service as well as Mus-lim soldiers. He was also an excellent tactician.
If you think the slaughter was bad in these conflicts Genghis Khan was far worse in his slaughter and it was not for any religious reason he did it.
