The enemy of your enemy is your friend!

There isn’t really much to say about the alliance with the Seljuk Sultanate of Rum. We don’t really know if they made an official agreement with the sultan, Kaykhaus I. Kaykhaus and the crusaders both campaigned against the Ayyubid rulers of Syria at the same time, and if they didn’t do this on purpose, it just seems like a weird coincidence, so we assume that they must have coordinated.

So how would crusaders and Muslims end up working together like this? It often seems like the crusaders, at least in the 12th century, were simply religious fanatics and were never willing to ally with Muslims no matter how beneficial it would have been. But they learned pretty quickly that alliances were important. For example, the crusaders in Jerusalem allied with Damascus in 1130 against the Zengi, the emir of Mosul and Aleppo. The various states in Syria weren’t united at the time, and that’s why the First Crusade had been successful. The crusaders realized a united Syria would be bad for them so they made alliances to help prevent that from happening. Nevertheless, in the end all the cities of Syria were eventually united under Zengi and his son Nur ad-Din.

In the 1160s, the crusaders in Jerusalem also tried stop Nur ad-Din from uniting Syria and Egypt. If that happened, the crusader states would be surrounded. Jerusalem intervened in Egypt and there were numerous alliances with (and against) the Fatimid rulers of Egypt. But eventually Egypt fell to Nur ad-Din’s general Saladin, who later destroyed the crusader army and recaptured Jerusalem.

That led to the Third Crusade. The crusaders from Europe were outraged when they learned that the Byzantine emperor was in contact with Saladin, but the Byzantines were skilled diplomats and they would ally with whichever side was most beneficial for them at the time, Christian, Muslim, or otherwise. The crusaders who lived in the east were already familiar with this kind of diplomacy.

Saladin’s empire fell apart when he died after the Third Crusade, so Egypt and the all the city-states in Syria were ruled by his family members, the Ayyubid dynasty. The Byzantine Empire was also temporarily destroyed by the Fourth Crusade, which was originally meant to attack Egypt, but ended up conquering Constantinople instead. So by the time of the Fifth Crusade, there was a “Latin Empire” in Constantinople, a Byzantine successor state in Nicaea in Anatolia, and the Seljuk Sultanate of Rum further east in Anatolia. Sultan Kaykhaus’ eastern frontier bordered on Christian states like Armenia and the crusader Principality of Antioch, as well as on the northernmost Ayyubid states like Aleppo. It was in the best interests of both Kaykhaus and the crusaders to prevent the Syrian states from uniting again.

The main part of the Fifth Crusade was directed against Egypt, but another wave, led by King Andrew of Hungary, sailed to Syria and campaigned against the Ayyubids there. Meanwhile, Kaykhaus campaigned against the Ayyubids of Aleppo. We don’t really know what the terms of the alliance were, or even if there was an official alliance at all, but as I mentioned, it just doesn’t look like this was a coincidence:

“Kaikhaus had ambitions of his own in the direction of Aleppo and Mesopotamia; so he supported the Frankish plan of a crusade against Egypt in the hope that this would tie the main Ayubid forces down in Egypt, leaving him with a fairly free hand in the north. The pincer attack began simultaneously on both fronts. At the end of May the crusaders arrived before Damietta, a strong fortress and the second most important port in Egypt; they pitched camp on the west bank of the Nile opposite the town. In June Kaikhaus attacked Aleppo but by August his campaign had already broken down in the face of al-Ashraf’s determined resistance. It ended too quickly to give the Franks the flank support they had hoped for.” (Mayer, 210-211)

One contemporary Muslim historian, Sibt ibn al-Jawzi, believed that Kaykhaus was the one who suggested this plan, but there's no account of it in any other Christian or Muslim sources as far as I'm aware. In any case, the general consensus is that the crusader campaign in Syria didn’t accomplish anything helpful for Jerusalem or for the crusaders in Egypt. King Andrew of Hungary returned home overland, passing through Kaykhaus’ territory in Anatolia, which is probably further evidence that they had come to some sort of agreement beforehand.

So, the crusaders who lived in the east had quickly learned the art of diplomacy and allying with Muslim states, and crusaders from Europe eventually figured it out as well. Today we tend to think of the crusades simply as religious wars waged by Christians against Muslims, but the reality was always more complicated than that. Crusaders and Muslims could easily ally with each other when they had a common enemy.

Sources:

Thomas Van Cleve, “The Fifth Crusade”, in A History of the Crusades, vol. 2: The Later Crusades, 1189-1311 (University of Wisconsin Press, 1969)

James M. Powell, Anatomy of a Crusade: 1213-1221 (University of Pennsylvania Press, 1986)

Hans E. Mayer, The Crusades, 2nd ed., trans. John Gillingham (Oxford University Press, 1972)

Apparently the idea of a crusader-Seljuk alliance was first suggested by Hans Gottschalk, Al-Malik al-Kamil von Egypten und seine Zeit (1958), but I haven’t been able to find a copy (and it’s in German anyway).