What basic principle is referenced in this quote?

checks and balances
separation of powers
federalism
popular sovereignty

the crusades are one of the most significant events in the history of europe and the middle east.   they were a series of religious wars carried out by christian crusaders from europe during the timeframe of the middle ages.   beginning in 1095 ce, the crusades saw european knights and noblemen travel to the middle east in an attempt to capture the holy land away from muslim people that had controlled the region for the previous centuries.   the term crusade means ‘cross’.   therefore, the europeans that became crusaders viewed themselves as ‘taking up the cross’.   in fact, many of the crusaders wore crosses on their clothing and armor as they made their pilgrimage to the holy land.

explanation: