An excerpt from an article by Thomas F. Madden at First Things (the emphasis is my own):
St. Paul said of secular authorities, “He does not bear the sword in vain; he is the servant of God to execute his wrath on the wrongdoer.” Several centuries later, St. Augustine articulated a Christian approach to just war, one in which legitimate authorities could use violence to halt or avert a greater evil. It must be a defensive war, in reaction to an act of aggression. For Christians, therefore, violence was ethically neutral, since it could be employed either for evil or against it. As Riley-Smith notes, the concept that violence is intrinsically evil belongs solely to the modern world. It is not Christian.
All the Crusades met the criteria of just wars. They came about in reaction attacks against Christians or their Church. The First Crusade was called in 1095 in response to the recent Turkish conquest of Christian Asia Minor, as well as the much earlier Arab conquest of the Christian-held Holy Land. The second was called in response to the Muslim conquest of Edessa in 1144. The third was called in response to the Muslim conquest of Jerusalem and most other Christian lands in the Levant in 1187.
In each case, the faithful went to war to defend Christians, to punish the attackers, and to right terrible wrongs. As Riley-Smith has written elsewhere, crusading was seen as an act of love—specifically the love of God and the love of neighbor. By pushing back Muslim aggression and restoring Eastern Christianity, the Crusaders were—at great peril to themselves—imitating the Good Samaritan. Or, as Innocent II told the Knights Templar, “You carry out in deeds the words of the gospel, ‘Greater love has no man than this, that a man lay down his life for his friends.’”
Read the entire article: First Things