The Crusades were a long and complicated series of conflicts that occurred not just in Jerusalem and the Holy Land, but also in Constantinople and eastern Europe, as well as in Spain and the Iberian Peninsula. The various sources on pp. 324-334 of the Human Record, show us some of the diversity of the players and reasons behind the Crusades. One important overarching theme is the idea of just war. The theory of just war (developed over time in the Middle Ages, from the 4th through the 12th c.) had three parts (referred to in Latin):
1) jus ad bellum, which concerns the justice of resorting to war in the first place;
2) jus in bello, which concerns the justice of conduct within war, after it has begun; and
3) jus post bellum, which concerns the justice of peace agreements and the termination phase of war.
How do the various writers deal with these concepts? For instance, how does Urban II's speech argue for the justice of fighting the Muslims in the Holy Land? How did Innocent III's letter to the Crusaders deal with the concept of just war against the Orthodox Christians of Byzantium? Do these arguments seem valid?