Fifth Crusade
The Fifth Crusade (1217-1222) struggled to attract the patronage of earlier crusades. Lacking definitive leadership, the Crusaders elected to travel to northern African to capture Egyptian territory of the Ayyubid's for the purposes of negotiating the return of Jerusalem to Christian control. Later, the Seventh Crusade would adopt a very similar strategy. Although the capture of Damietta boded auspiciously, the crusade army was undone by a reliance on prophecy that had them heading west to Cairo. The rising of the Nile River prevented the army from advancing and they summarily captured as they retreated to Damietta.
The story goes something like this:
In 1222, intelligence relayed from Bohemond IV, ruler of the crusader state of Antioch, to Jacques de Vitry, preacher and crusade propagandist, reaches crusaders in Damietta. The intelligence, a report written in Arabic obtained from traveling spice merchants in Antioch, details the westward military progression of a certain King David, purportedly the great-grandson of the famed Prester John, a military leader who, rumor has it, has systematically destroyed Muslim armies in the east.
Jacques has the report translated immediately. Buoyed by prophecy and heedless of local conditions, the crusaders at Damietta decide to invade Cairo immediately to fulfill the prophecy, rejecting an agreement with the Sultan Al-Kamil that would have given Jerusalem back to the crusaders in exchange for Damietta. The Nile rises, turning the invasion of Cairo into defeat. The armies of the Fifth Crusade surrender to the Sultan of Egypt, Al-Kamil, Saladin’s nephew, a few weeks later.
This King David was not Prester John but, in fact, referred to Genghis Khan.