First time answering on this sub so here goes.

So i view this question as having multiple answers due to the complex nature of medieval Europe and the relationship between the church and "politics".

The initial call for assistance was from the head of the Byzantine Empire Alexios Komnenos. Alexios appealed for help from the west due to the encroachment of Seljuk Turks into his empire, the rich province of Armenia had fallen to the Turks and the Byzantine emperor could see the danger his nation was in.

Although initially this was not seen as a call to crusade more as a traditional call for help from one power to another.

Pope Urban II responded to this plea and sounded the call for crusade, although the idea of a holy war was a new concept in Christianity, so the crusades were sold more as a armed pilgrimage instead of a traditional war.

In regards to your question though, i would recommend looking more into the aftermath of the First World War with the sykes picot agreement. I would argue that this is where the majority of the issues with the middle east have stemmed from.

Thank you, that helped me a lot!