Islam was only 300 years old when the crusades began, so how could it have been a “1000 year war against Christendom?”
Islam was confined to the Levant (in the middle east, at least), colliding with the Byzantines in Anatolia. So, how was this a threat against countries such as France, England and the Holy Roman Empire? Also, the crusades were an effect of the emperor in Constantinople requesting a contingency of knights and footmen to aid imperial troops; the last thing the emperor expected was entire armies (and it is interesting that when those armies arrived at the gates of Constantinople they were NOT let in for fear that they were there to conquer it).
When Tancred the Norman burst upon the temple mount during the first crusade, he and his men smashed their way in the Dome of the Rock and stripped it of "more than forty silver candelabra, each of them weighing 3600 drams, and a great silver lamp weighing forty-four Syrian pounds, as well as a hundred and fifty smaller silver candelara and more than twenty gold ones, and a great deal more booty." The Moslems fled into the other great structure on the Mount, the al-Aqsa Mosque. There they surrendered, promising a great ransom for their lives and accepting Tancred’s banner as the symbol of his protection. Before the sun set, Tancred’s flag, hoisted by Moslems, was flying over the most sacred site in Jerusalem. The following morning the Crusaders re-entered the al-Aqsa Mosque and slaughtered every Moslem sheltering there.
Does that fit your idea of “just war?” Was this done for the glory of God?