How could the 1st Crusade be defeated without seizing Jerusalem?
And, if the 1st Crusade fails to take Jerusalem, would armed Crusades be attempted later in the following years, decades or centuries of the medieval era?
Or would the initial failure forestall Crusades-mania and enthusiasm, leaving the Muslim states of the Levant free of European invasions for over five hundred years, like they were between OTL's 1291 and 1798?
And enough about the impact on Europeans and their plans.
How is Levantine and Middle Eastern development changed by a lack of successive Crusades?