This fervor and drive allowed for the Crusaders to take Jerusalem in 1099 and subsequently, carve out the Latin kingdoms of Jerusalem, Tripoli, Antioch, and Edessa. These successes allowed for the Crusaders to establish their footprint in the Levant but would not endure beyond a few hundred years. The reasons for their failure could have very well been their reasons for a lasting presence and success in the Middle East had they properly adapted to the circumstances of their situation.
First, no culture or civilization can survive let alone thrive if there is not a sustainable population that is growing. The crusades themselves, while there were non-military zealots and the like that did make it to the Holy Land, were mostly comprised of military forces. The evolving decades saw the development of the various military orders such as the Templars and Hospitallers who would become the defenders and stewards of their various lands and though there were some efforts to attract settlers, the lack of a concerted effort to properly do so, would seriously undermine the manpower, economic and food production resources of all the Crusader kingdoms.
The use of Turcopoles, ‘indigenous mercenaries who served the western knights as mounted archers and other types of light cavalry’ were the right kind of troops to satisfy both their military manpower and general population needs. These Turcopoles were possibly native Christians, the result of Christian and Muslim unions or a combination of both (there is some academic debate as to this) but their value could not be understated. The Latin Kingdoms could have helped their longevity by building upon the integration and use of Turcopoles beyond just that of the army, by offering land-grants, societal status, and the like, to foster more willing migration to their various kingdoms. This way, the families of the Turcopoles would be members of the various kingdoms and become more vested in its success and survival and in turn, be consistent contributors to said success through food production, taxes paid and the like. In the harsh environment of the Middle East this would have provided a better standard of living and therefore would in turn, attract more people to them, thus sustaining and growing the kingdoms (and providing more willing military manpower to protect their homes).