History of Christianity
The Crusades were a military conflicts conducted by Christian knights for the defense of Christians and for the expansion of Christian domains.  Generally, the crusades refer to the campaigns in the Holy Land against Muslim forces sponsored by the Papacy.  There were other crusades against Islamic forces in southern Spain, southern Italy, and Sicily, as well as the campaigns of Teutonic knights against pagan strongholds in Eastern Europe, and (to a much lesser extent) crusades against Catharism or other Christian heresies.