The princes' crusade refers to the Third Crusade (1189-1192), which was a major military campaign initiated by European monarchs to reclaim Jerusalem from Saladin, the Sultan of Egypt and Syria. This crusade was marked by the involvement of several prominent leaders, including Richard the Lionheart of England, Philip II of France, and Frederick I Barbarossa of Germany, highlighting the political and military aspirations of these rulers as they sought to assert their dominance and gain glory through religious warfare.