Why did European Christians launch the crusades?

This is a very complex question to answer, but here is an attempt to make it quick to read.

Up to the time the Crusades began in the late 11th century, the Holy Land had been in Christian (Greek Orthodox) hands under the Byzantine Empire for most of its history. In the late 600's and early 700's Islam spread rapidly with a sWord from Arabia across North Africa to Spain. For the next few hundred years the Byzantines fought the Muslims keeping their advance in check to varying degrees.

Their primary two Muslim enemies were the Turks and Egyptians. The Egyptians eventually conquered much of the Holy Land whereas the Turks began to push into Anatolia in, of course, modern-day Turkey. Not long before the 1st Crusade was called, the Byzantines suffered a terrible defeat against the Turks and were in a tough situation to continue defending against the spread of Muslims into their lands. As a primary reason, this is why the Crusades were first called. Some also argue that the Crusades were started to help stop Europeans from killing each other by creating a common enemy in the "Saracens."

However, the Crusades were given a religious tone because the man who called for their initiation (Pope Urban II) was the religious leader of Europe and similarly the only European leader recognized by every nation (if it was just the king of France who called a crusade there would probably only be French crusaders taking part).

Finally, the tone of the crusades was changed by the fact that for every warrior who fought for religious conviction, there were probably 3 or 4 that were fighting for land. For example, the leaders were: Raymond, Godfrey, Bohemond, Tancred. Raymond was probably the only leader fighting purely for religion whereas Godfrey was a mix. Bohemond and Tancred quickly abandoned the progress of the Crusade once they got land (Antioch and Galilee respectively) while Raymond and Godfrey were the two who actually took Jerusalem.

Attempt to make this quick: FAILED. Hopefully this made sense though.