This refers to the Siege of Ma’arrat an-Numan in late 1098-1099. The crusaders were stuck at Antioch for most of 1097-1098, but once they defeated the Turks and gained control of the city, they continued south towards Jerusalem. In November 1098, they reached Ma’arrat, which they besieged and captured in January 1099.

“At some point during this activity…an indeterminate number of soldiers ate from the flesh of enemy dead.” (Rubenstein, pg. 526)

The fact that this happened is not really in question, since the only two chroniclers who were actually present mention it, and apparently it was so well known that all the other First Crusade chroniclers mention it as well, although the details are never quite the same.

Some of the accounts originate with the Gesta Francorum, a chronicle by an anonymous knight who was present with the Italian Norman contingent. From the Gesta, it was copied by other authors back in Europe, including Peter Tudebode, Guibert of Nogent, Robert the Monk, and Baudri of Bourgueil.

Raymond of Aguilers was also there, with the southern French contingent. Both Raymond and the Gesta mention that the cannibalism occurred after the siege was already over.

There is a a separate account in Fulcher of Chartres, who was on the crusade, but was not present at Ma’arrat. He apparently heard that the cannibalism happened during the siege

Other chroniclers back in Europe also mentioned it, independent of any of these accounts. Albert of Aachen wrote that they ate people, but even worse, also dogs. Ralph of Caen has a particularly lurid account where they roast Muslim babies on spits and all sorts of other crazy details. You sort of wonder if years later, some old crusaders were telling him stories and exaggerating, just pulling his leg…

Later 12th century authors like William of Tyre (who was the official historian of the crusader Kingdom of Jerusalem) say it happened at Antioch in addition to Ma’arrat. The Byzantine princess Anna Komnene heard these stories as well, but she places the cannibalism at the beginning of the crusade, at the Siege of Nicaea in 1097. Meanwhile, the Chanson d’Antioche, a poem about the crusade, is clearly not a reliable historical source, but it places the cannibalism at Antioch and treats it as a hilarious joke meant to terrify the Turks. The crusaders feast on some Muslims, and Godfrey of Bouillon shows up and offers them some wine to wash it down.

In short, all the Christian sources agree that it happened. They differ in the details, which is probably due to different crusaders informing different authors of different things, but there doesn’t seem to be any doubt that it really happened. Interestingly enough, however, Muslim sources never mention it…that might be because there just aren’t any strictly contemporary Muslim sources like there are Christian ones, but at the same time, none of the sources from decades (or centuries) later thought it was worthy of mentioning, if they even knew about it. It’s possible that they simply never knew of it, especially in the cannibalism took place after the siege, when everyone who could have informed the Muslims was already dead (and being eaten).

It’s also highly likely that, even if cannibalism really did occur, the descriptions of it are more of a literary device. Cannibalism was probably intended to be something so extremely transgressive that no rational medieval person would ever do it. If some crusaders resorted to it, it was to show how desperate they were. The accounts in the chronicles are almost all written by church-educated authors who would definitely have known about similar incidents in the Bible, so they could easily use Biblical allusions in their descriptions

Rubenstein’s conclusion:

"A possible and plausible reconstruction is that the first acts of cannibal-ism happened during the Crusade’s initial experiences of scarcity and siege near Nicea, where Anna Comnena places it. It almost certainly recurred at Antioch, where William of Malmesbury, William of Tyre, and the Chanson d’Antioche have it. By the time a similar desperation had spread among the army at Ma‘arra, some of the soldiers must have recognized its potential utility and, hoping to drive the defenders into a quick surrender, made a spectacle of the eating, and made sure that Muslims were the only ones eaten. That is probably why, for the earliest chroniclers, the cannibal-ism at Ma‘arra was the most memorable instance of what was likely a periodic response to famine: it was the most public, the most deliberate, the most celebratory instance…” (Rubenstein, pg. 550)

So, in short, according to all the Christian chronicles it happened; they simply differ in the details; and no Muslim sources mention it.

Sources:

Jay Rubenstein, “Cannibals and Crusaders”, French Historical Studies, vol. 31, No. 4 (Fall, 2008), pp. 526-552

Thomas Asbridge, The First Crusade: A New History (Oxford University Press, 2004)

Jonathan Riley-Smith, The First Crusade and the Idea of Crusading (University of Pennsylvania Press, 1986)