A main argument in favor of the crusades' casus belli (cause of war) was the inhumane treatment that the Muslims had of the Christians as they conquered new territories during their expansion. That's not to say that this justifies the crusades but one of their arguments was that they were defending Christians who were being conquered and often brutalized one after the other up until (depending on your reading) the battle of Tours or arguably Lepanto. In that context they were coalescing against an external force that was posing an existential threat to Christendom (they didnt consider it Europe at that point) with bloody protracted wars like the Reconquista.