Crusade

In Medieval 2: Total War, crusades are "Holy Wars" fought by the Catholic factions against non-Christians in order to seize their lands. They are called by the Pope but other Catholic factions may also request him to call a crusade. All Catholic factions can join a crusade as long as they aren't excommunicated. Crusading armies have increased movement points but their soldiers might desert if they do not come closer to the crusade's target every turn.

The Muslim version of this is a Jihad.