My professor mentioned this the other day and reading some of the threads posted yesterday reminded me to see what /r/askhistorians thinks.
I've always assumed the Crusades were a result of Urban responding to a Byzantine plea for assistance, but clearly it is not as clear cut as I previously believed.
hi! just wanted to double-check, did you follow the link in yesterday's post to the FAQ section Motivations for the Crusades? It contains several in-depth posts, starting with /u/Ambarenya's lengthy overview here
In the event the FAQ Searocksandtrees doesn't satisfy you, I've been doing a lot of work/research with the crusades recently and I'd like to add some things.
As many articles/posts/whathaveyou say, the Byzantine Emperor Alexios Komnenos's plea to Urban II for aid was certainly a sort of spark to the tinder that was building in the East. The Seljuks were rapidly expanding, and with the capture of Jerusalem (albeit from other Muslims) seemed to have caused many eyes to look to the East.
It was around this time and especially during/after the First Crusade that the idea of "warrior monk" started to come about, and it's an idea that coincides with the "brothers in Christ" idea, where the East and West started to see more commonalities with each other. Urban was likely trying to stop the continuous infighting among Christians in Europe, and there was hardly a better way than helping fellow Christians (especially with the promise of salvation).
Bernard of Clairvaux's In Praise of the New Knighthood, which was a letter written on behalf of one of the founders of the Templars, Hughes de Payens, where (later, Saint) Bernard lets everyone know that the idea of "warrior monk" actually does make sense, that it's all good with God if you kill infidels, and that you'll receive salvation.
While it's addressing the Templars in particular, it applies to the general idea of a "crusader" in that violence in the name of God was acceptable.
So I'd say it's definitely a combination of internal and external, since Urban was at least partially motivated by ceasing the infighting of Catholics/mending relations between the Catholic/Orthodox churches and the fact that the East was in genuine trouble.
I suggest checking out Jill Claster's Sacred Violence or, for a shorter read, the 2013 edition of Thomas Madden's The Concise History of the Crusades.