Body: "History of the Crusades" is the examination of Christendom's fight to rescue the Holy Land from Muslim invaders.
A few notable experts of "History of the Crusades" include Thomas F. Madden (Director of Saint Louis University's Center for Medieval and Renaissance Studies), Jonathan Phillips (an expert from the University of London), Paul Cobb (Notre Dame).
Some subfields in "History of the Crusades" include centuries of Muslim invasions into Christian lands before the Crusades, why the Holy Land is important to three religions, the average soldier, etc.
Some motivations are Better understand medieval history, gain insight into modern conflicts.