The Crusades were a series of military expeditions sent by various European kingdoms beginning in 1096. They were directly initiated or sanctioned by the Catholic Church. They aimed to recapture Jerusalem from Islamic states, persecute heretics and Jews, and to gain converts. The Crusades ended in 1291 with defeat for the Christians. 
"Let this then be your war-cry in combats, because this word is given to you by God. When an armed attack is made upon the enemy, let this one cry be raised by all the soldiers of God: It is the will of God! It is the will of God!"
- — Pope Urban II, recorded by Robert the Monk 
After the split of the Roman Empire, Jerusalem was under control of the Christian eastern Byzantine Empire from around 330 until its capture by Islamic Arab caliphates in 638. From this time until its capture in the First Crusade in 1099, it was under Muslim control, with Christian and Jewish communities being tolerated as effectively second class citizens. However, Muslim toleration of pilgrims diminished as the Byzantine empire expanded into the Levant. The first crusade was in support of the Byzantine empire and in response to reports of the killing of Christian pilgrims.
The extent to which the Crusades was motivated by religious extremism, holy war, recapturing formerly Christian lands or imperialism is controversial.
- "For starters, the Crusades to the East were in every way defensive wars. They were a direct response to Muslim aggression — an attempt to turn back or defend against Muslim conquests of Christian lands. "
- "Did the crusaders view their war in these terms? Was it a defensive war? The answer is a resounding "no." The crusaders had one goal in 1096: to capture the Holy Sepulcher, the tomb of Christ, in Jerusalem. [...] The First Crusade, then, was not about turning back centuries of Muslim expansion. It was about seizing control of sacred landscapes. "
- "Within an age of such intense religiosity the city of Jerusalem, as the place where Christ lived, walked and died, held a central role. When the aim of liberating Jerusalem was coupled to lurid (probably exaggerated) stories of the maltreatment of both the Levant's native Christians and western pilgrims, the desire for vengeance, along with the opportunity for spiritual advancement, formed a hugely potent combination. "
Most probably, the main motivation of the Crusades was not to recapture territory on behalf of the Byzantine Empire because the campaigns did not attempt to recapture territory other than to fulfil religious or imperialistic goals (most of what became modern Turkey was still controlled by Muslims), and any territory that was captured was usually held as separate Crusader states. If it was a defensive war, the crusaders would have fought alongside the Byzantine forces.
Although atrocities were often committed in the warfare of the Middle Ages, the Crusades were particularly brutal, perhaps because of religious extremism and the belief that the war was sanctioned by God. This includes the massacre after the Siege of Antioch in which both Muslim and Christian residents were killed, and the Siege of Jerusalem in 1099 in which many of the defeated Muslim and Jewish defenders were killed.
The Baltic Crusades or Northern Crusades were for the sole purpose of converting the Pagans of north-eastern Europe. Many people were killed for refusing to convert to Christianity. The Baltic Crusades are a true example of the lack of morality in Christianity.
Harm religion causes
- Main Article: Religion is harmful
The history of the Crusades and the Islammic Jihads illustrate the harm the more aggressive types of religious system can cause.