In his article on Medievalists.net Peter Konieczny (...) presents the strong case that before and at the start of the First Crusade "an alliance existed between the Crusaders and the (muslim; TN) Fatimid rulers of Egypt (...) By the latter decades of the eleventh-century, these states (Byzantium and Fatimid Egypt; TN) had only known peace with each other in living memory, which is remarkable for the medieval world.
The Fatimids and the Byzantines had good reason to be allied with each other – they both were threatened by the Saljuq Turks, who by the mid-11th century were establishing an empire that stretched from Central Asia to the Mediterranean. They brought war against the Byzantines and Fatimids (...). It would be the Saljuqs who would conquer Jerusalem from the Fatimids in 1071 – and it was also they who started preventing Christian pilgrims from going to the Holy City. (...)
Several accounts about the First Crusade note that around February 1098, when the crusaders were besieging the city of Antioch, a delegation arrived by sea from Egypt. (...) at this point Jerusalem was still in the hands of the Saljuq Turks. (...) It was during this visit that crusaders engaged in battle with Ridwan of Aleppo, dated to February 9th (...). While other accounts acknowledge that the Egyptian delegation was present for this victory, Albert of Aachen is the only source that states they fought side-by-side against the Turks. (...)
The notion that the Crusaders and Fatimids had made some sort of alliance also helps to settle a question that has puzzled historians of the First Crusade: why did the crusaders, after defeating the Turks at Antioch in June of 1098, decide to wait until the beginning of 1099 to resume their march towards Jerusalem? Other factors have been suggested, such as turmoil among their leadership and questions of what was to be done with Antioch, but so could have been the idea that the crusaders were waiting for the Egyptians to complete their part of the deal.
In August of 1098 an Egyptian army marched to the gates of Jerusalem, and began a siege of the city. It did not take long for the local Saljuq garrison to surrender. Soon after, the Egyptian forces (to the opinion of the well known chronicler William of Tyre; TN) (...) seemed to imply that they were conferring a great favor on the Christians by allowing unarmed pilgrims to go to Jerusalem in groups of two or three hundred and return in safety after completing their prayers.The leaders of the Christian forces regarded this message as an insult (...) to which they would not consent. (...)
In early 1099 the crusaders began what was essentially a dash towards Jerusalem, hoping to catch the Fatimids unprepared. In fact, the city of Jerusalem was currently being demilitarized (by the Fatimid Egyptian occupation; TN) with its defences being torn down, which offers more indication that the Egyptian rulers were under the impression that they had a fairly strong alliance with the Crusaders. By the time they could react, Jerusalem would already be under attack, falling on July 15, 1099. (...)
The attack and capture of Jerusalem proved to be a shock to Egypt and the wider Islamic world, and crystallize a view among Muslim writers that the Crusaders, and Western Europeans (Franks as they would call them) were untrustworthy and duplicitous. (...)
(...) it suggests that the ideas that crusades was directed against Islam itself is somewhat misplaced – the initial threat happened to be the Saljuqs, who happened to be Muslim, and that they were prepared to work with other Muslims in order to defeat them. While the crusades would later evolve into a war between religions, it did not necessarily start out as one. (...)"
This blog quotes extensively from the paper"Were Christians and Muslims Allies in the First Crusade?" by Peter Konieczny, published on medievalists.net. Read the full paper for all important details. Source of illustration