The Crusades were defensive in nature.
The historian Thomas F. Madden has argued that the Crusades were fought by Christians for primarily defensive reasons. (*)
The Crusades were initiated after Muslims had invaded Europe from the west into the Iberian peninsula and France; from the south into Italy; and from the east into eastern Europe. The Crusades began after the Muslims had violently conquered what had been the predominatly Christian areas of North Africa and the Middle East. The various abuses and excesses perpetrated by the Crusaders are regrettable. The Crusades were ultimately unsuccessful in taking and keeping key areas of the Middle East, including Jerusalem. Yet, they were largely successful in slowing the advance of the conquering Islamic armies, allowing Christian Europe to prosper long enough to reach the Renaissance.
Howard Owens has more. (†)
Update: 6 December 2003. Edited for phrasing.