The crusades were wars sanctioned by the Catholic church during the medieval period. There were many crusades although many people only know about the crusades in the Levant. It was these crusades that were the first crusades that happened so this is understandable.
How did the crusades start?
Unfortunately, the reasons for the start of the crusades cannot be boiled down to a single incident or action. While the First crusade as an action can be boiled down this far the reasons why the crusades started lay within the events that were happening throughout Europe and the Byzantine Empire as well as the events in the Middle east.
The first crusade itself was a direct response to a request for aid from the then Emperor Alexius I. The Byzantine Empire had been dramatically changed from its time as part of the wider Roman Empire. After the collapse of the Western Empire, the Eastern Empire had tried to stabilise itself as a major power. Over the intervening 600 odd years between the collapse and the first crusade, the Byzantine Empire had had many ups and downs. Although, by the time of Alexius, it had stabilised. Even though Alexius had managed to stop the empire from declining he still had a lack of troops to combat those on his borders and also to reclaim the land that had been lost during the Islamic expansion.
Historians, such as Thomas Asbridge, have suggested that Alexius was expecting a mercenary force that he could use to bolster his forces. This makes a lot of sense if the Byzantine military as a whole is considered. Throughout much of its history, it had used mercenaries to bolster its forces and frequently had the gold to pay for them. Alexius I did indeed have enough gold for such a purpose too. Western and particularly Northwestern Europeans had been used to fill the ranks of the Varangian Guard and were considered one of the elite units of the Byzantine armies.
While this core fact is considered as one of the founding actions of the crusade movement the reasons for the call for aid were deeper than simply to take back Jerusalem. The Byzantine Empire had controlled much of the Eastern coast of the Mediterranean and within that area, there were many large cities that were both prosperous and wealthy which would have helped to support the Empire.
Within Europe, it was the Pope who started the crusading movement. During a large service in November 1095, Pope Urban II decided to put forward the idea of forming a Christian army to retake Jerusalem. This idea is obviously linked to the correspondence received from Alexius I and on the surface could simply be a call for an army to directly help the Byzantine Emperor. The problem with this is that it misses the context within Western Europe at the time. The Church and Urban himself wanted to increase their power. Having an army that is directly run by the Pope and his officials would indeed serve this purpose. While the merits of the actions taken by Urban II can be debated it would be foolhardy to suggest that he did not have an agenda in calling the crusade. When one considers that Jerusalem had been under Islamic control for roughly 400 years it could be considered that the retaking of Jerusalem was not due to the Muslims being in control of Jerusalem but for other reasons. These reasons will be explored in another post as they are not directly relevant here.
So, in conclusion, How did the crusades start?
Pope Urban II called the First crusade in November 1095. This was at least in part due to a request for aid from the Byzantine Emperor Alexius I for troops to help him protect his Eastern borders from the Turkish kingdoms. The Europeans who heard this helped spread the message gathering troops and equipment to set off in 1096.