I keep hearing that the pope started it and that it was a response to Muslim aggression. But which one is true? And is either true?
Hi! Not to discourage further discussion, but I have written previously on traditions of holy war and sacred violence that underpin the crusading ethos as well as how the seizure of Jerusalem was rationalized as an act of defense by early 12th century ecclesiastics-- even if their narrative of a cosmic struggle between good and evil does not reflect reality. You may find those answers helpful for context. Outside of that, you may want to try your hand at some of these other answers that tackle the subject, particularly with regards to popular claims about crusade::
You might also want to check out the FAQ section Motivations for the Crusades, in particular the post:
Both of those are semi-true, though the Muslim aggressors were not the ones who the Crusaders attacked, because not all Muslims are the same.
Tldr; The Greek Emperor wanted to take back Anatolia (most of modern Turkey) which he'd lost to the Turks decades before, he asked the Pope to send help, instead the Pope sent a huge army of warrior-pilgrims whose aim was to drive the Muslims out of Palestine and take control of the pilgrimage sites there, such as Jerusalem.
The trigger for the crusades was a letter sent from the Byzantine Emperor to the Pope, asking for help in taking back lands lost to Muslims. This was a few decades after most of Anatolia was lost to the Turks (hence why it's now called Turkey), and more than 300 years after Syria and Palestine were lost to the Caliphate. Immediately after the Battle of Manzikert, when the Turks beat the Byzantines, the Emperor had sent a similar letter, but the Pope did nothing. This time, however, there was a new Pope, who was much keener on helping. So, the Pope declared the first Crusade, making really inspiring speeches, and sending out preachers to inspire others. These focused a lot on taking back the Holy Land (which, it's important to note, was not actually ruled by the Turks). Far, far more people (both peasants and knights) turned up in Greece than the Byzantines expected. Tensions led to the Crusaders and Byzantines not working together very well. One other issue the Pope brought up was how the Muslims had stopped Christian pilgrims from going to Jerusalem. This is sort of true: Jerusalem was on the front lines between the Seljuq Turks (Sunnis) and the Fatimid Caliphate (Shi'a). Warfare between them had made it difficult and dangerous for pilgrims to visit Jerusalem. Not because the Muslim leaders were deliberately murdering Christians or keeping them out (though local Muslims might have attacked pilgrims), but just because it was a war. Anyway, the Crusaders marched into Anatolia, fought the Turks, but eventually carried on through Anatolia and down to their real target, the Holy Land. They beat the local rulers, and formed the Crusader States. They weren't supposed to do this: they'd sworn oaths to give all land seized to the Byzantine Emperor (the original point of the whole thing). But as the Byzantines hadn't supported their battles, they felt it was justified to rule themselves.
So, the Crusades were in response to Muslim aggression, decades later, and targeting different Muslims to the ones who'd actually attacked. Of course, the Crusaders weren't that bothered by this (once they realised the Turks and the rulers of Palestine weren't on the same side: they had a very poor understanding of both Islam and Middle Eastern politics). To them, they were taking back land conquered by Muslims, hundreds of years before. They saw it as a battle between civilisations: the fact that the Muslims who ruled Palestine were not the same Muslims as those who'd recently been invading Byzantium was pretty irrelevant: they lumped all Muslims together.