This is a bit of a complicated question, while the church SAID that they where trying to reclaim the holy land, but it was actually mostly started for political reasons.
Muslims and Christians actually got along fairly well in the 8th century. But with the Battle of Manzikert, the tension increased with the empire. Not to mention at this time the holy roman empire (HRE) had a large controversy, where people wondered if the catholic church of the HRE had the right to appoint clerics and other church officials.
This all lead to an increase in care in religion and with propaganda of a "just war" being spread around by the Papacy the war was inevitable.
