The Crusades were of course a series of Christian invasions of Muslim held lands in the Levant. All of the Crusades were called upon by the Roman Catholic Church and the purpose was to replevin Jerusalem for the Christian faith. The Seljuk Turks held the area tightly since 638 in the Rashidun Caliphate.
While there were nine major Crusade movements in two hundred years, the truth of the matter is far more complicated. There were some smaller Crusades in between, and there were miniature kingdoms that formed. The County of Edessa, in Anatolia, was declared in 1098 and lasted about 50 decades when the Zengid Turks sacked it in 1144. Archbishop Hugh rode to Rome and walked away a Papal Bull for a second crusade.
The Crusaders who lived in Edessa had to defend themselves against the Saraçen warriors , a force that caviled the Crusader force down over the years. So they were at constant war. For the religous fanatics on both sides, this was heaven.