Crusades
mass military expeditions and wars initiated by the Roman Catholic Church in the High Middle Ages
Crusades were military campaigns sanctioned by the Catholic Church during the Middle Ages, to expand the realm of Christianity or to put down heresies within it. They have also inspired numerous dramatic works in the literature of Christian Europe.
The Crusades most commonly refers to the nine crusades to the Holy Land from 1095 to 1272; those are the topic of this article. Other crusades include Northern Crusades to Balticum and Finland, the Abligensian Crusade against the Cathars of what is now southern France, the Reconquista taking the Iberian Peninsula back from the Moors, and several crusades against the Hussites of Bohemia.
While the headline summation is "Roman Catholic Europeans going to the Holy Land to conquer" and it is relatively well known that a speech by Pope Urban II in 1095 launched the first crusade, the actual history is much more complicated. Pilgrimage to the sites of the life of Jesus of Nazareth had been a Christian tradition at least since the trip of Emperor Constantine's Christian mother to Jerusalem in the 4th century where she claims to have found the "True Cross of Christ" (splinters of which would later add up to a veritable forest of crosses). See Jesus Trail for some of the pilgrimage routes.
With the Byzantine Empire fighting an on again off again war against Muslim adversaries to the East, the rulers in Constantinople started toying with the idea of asking the Christians to their west to aid them in defeating the "heathens" to their east. Reports of pilgrims being harassed and even a desecration of Holy Sites further fueled the desire by Europeans to take revenge for real or perceived wrongs.