Muslim armies had conquered Syria, Persia, Palestine, Egypt, North Africa, Spain, Sicily, Southern Italy, Cyprus, Rhodes, Sardinia, Majorca, Crete, and Malta which had been some of the most heavily Christian areas in the world. They attacked Rome and came within 100 miles of Paris before being pushed back.
Thousands, and possibly millions, of Christians died during this drive to eventually bring the entire world under Islam.
The First Crusade was launched in 1095 by Pope Urban II to check the advance of the Muslims and regain control of the city of Jerusalem and the Holy Land.
If this defensive war was not fought then we would probably all be Muslim today.
I am sure that some atrocities were committed by individuals of both sides during this war but by most people's judgment this was a just war.
For more information, see:
+ The Crusaders by Régine Pernoud
+ The Decline of Eastern Christianity under Islam: From Jihad to Dhimmitude (1996) by Bat Ye’or
+ God’s Battalions: The Case for the Crusades (2009) by Rodney Stark
+ Islam at the Gates (2008) by Diane Moczar
+ Seven Lies About Catholic History (2010) by Diane Moczar
+ Wikipedia: http://en.wikipedia.org/wiki/First_Crusade
With love in Christ.