I believe that I was correct in my reasoning behind the Crusades.

The First Crusade was started after the loss of the Battle of Manzikert, which cost the Byzantine Empire what is now modern Turkey. Alexius I appealed to Pope Urban II for mercenaries to assist in the fight against Muslim advances, and Urban sent a large force not only to protect Byzantium, but to retake Jerusalem. There were other contributing factors of course - the excess of military forces in Europe, the religous zelousness of the period, and other causes - but the actual trigger of the First Crusade was in fact the request for aid from Alexius, and the later Crusades owed their existance to this act.