Reasons for the Outbreak of the First Crusade

Why was the first Crusade launched?
The outbreak of the first crusade in 1095, played an important part in the future of medieval history, we can argue many different reasons and causes for the first crusade and it is impossible to finally put it down to just one reason. The outbreak of the first crusade followed a series of different triggers for example religion or economic gain but historians will always argue which factor was the most vital in the causation of the first crusade. It can though be said that pope urban the second played a major part in the outbreak of the first crusade and his decisions led to the start of other triggers.
The development of the concept of crusading first developed with St Bazil a byzantine saint when he came up with the concept of a just war. he said that you could justify a war if it was in self defence, he said that sometimes when a army knows they are going to be attacked they can use a pre- emptive strike and attack their enemy before they have chance to attack, this makes it a just war. St Augustine of hippo then expanded upon this theory by saying that you should imagine that the roman empire is like heaven, inside the walls of the empire are the faithful Christians like in heaven those inside have been saved, however outside are the non Christians who like those who are outside of heaven need to be saved, he went on to say how the roman empire is part of gods divine plan and is like a state of god on earth except the roman emperor is charge like god, therefore it was the job of the Emperor to ensure those on the outside of the city were saved and converted to Christianity. If these people had to be forced into Christianity it would be in the name of god, Pope Leo IV then expanded further on the idea of the crusades by developing the theory of plenary indulgence. Now there was a justification for Christians to crusade making it acceptable and giving them an end reward.
The period in history prior to 1095 was an...