What were the crusades, why did they happen and what were the effects of them

The crusades where a string of several attacks by Europeans on the Muslim controlled Jerusalem. They first was declared holy by Pope Urban the 2nd who had sent the Europeans in the first place. Originally it was a political move made by the Catholic church in order to prevent the Catholic fiefdoms from fighting each other. They continued because of the three He's. God, Gold, and Glory. The crusades caused unrest in the Catholic church and were one of the first causes of the reformation.