We must be careful not to over simplify this topic. The whole thing is so complex, and with so many separate events and participants that it cannot even possibly be discussed in one thread.
The crusading era was more than just the middle east, and much more than just fighting Muslims. There were crusades into eastern Europe and against religious dissenters in the west. There was the long running crusade to conquer Spain, and the conquest of Sicily and Malta could probably be considered likewise.
But most people think automatically of the Crusades in the east.
By the time of the Crusades, the Muslims had been resident in the Holy Land for some three hundred years at least. They were not “invaders” at that point, many (if not most) of them were descendants of the original inhabitants of Palestine, with some Christian and Jewish ancestry as well as some Arabic.
The actual events that touched off the Crusades were persecution by a Caliph Hakim, it lasted ten years and resulted in a lot of destruction of churches. Especially notable being the fate of the great church of the Resurrection (called the Holy Sepulchre in the west).
By the time of the launching of Crusades it was all over, another local regime was in place and the situation was fairly normal, churches were being rebuilt, etc.
I think that the original motivation, supplied by Pope Urban, was noble enough, but the people who (by and large) answered the call were not the flower of Christianity, and they were (by and large) very greedy, cruel and triumphalistic.
Pope Urban responded to a request for mercenaries by the Byzantine emperor. The territory of the Holy Land was Byzantine-Roman/Christian before the coming of the Muslims, he hoped that a supply of mercenaries from the west supporting his own professional military would make it possible to drive the Muslims out of important cities and (hopefully) down the Mediterranean coast to Egypt.
The Crusades were more than he asked for, and even after vowing to serve under the Byzantine banners the Crusading leadership acted quite independently, eventually abandoning all pretense and declaring themselve sovereign powers.
Their own natural allies should have been the local Christians, and they initially received cooperation from these people. They made a terrible mess of it, alienating everyone.
Michael