Crusades
The crusades started as a fight for control over the Holy Land.  This is the land now called Israel.  The Holy Land is very important for the three major monotheistic religions: Islam, Judaism, and Christianity.  Muslims believe the Prophet Mohammed visited the Holy Land and rose to Heaven from Jerusalem.  Christians believe Jesus was born, crucified, and resurrected in the Holy Land.  The Dome of the Rock, the Western Wall, the Temple Mount, and the Mount of Olives are all located there.  So are many other important religious sites.