In the spring of 1242, after nearly a year of occupying the Danubian plains, Batu Khan abruptly withdrew his forces from central Europe. Latin chronicles from the period express both horror and dismay around the Mongols’ mysterious departure. Ivo of Narbonne relates how the sudden retreat caused even ‘more fear for all those who witnessed it’ than the initial attacks; while a German annalist bluntly concludes that ‘only God himself knows’ the reason for it. Amidst this backdrop of fear and uncertainty, Pope Innocent IV appointed John of Plano Carpini to lead a diplomatic mission to the court of the Great Khan with the hope of finding some ‘remedy against the Tartars’. John embarked on the journey in 1245, passing first through the conquered Kyivan Rus’ principalities before reaching Guyuk Khan's court near Karakorum. When John returned to Lyon in 1247, his advice to the pope was unequivocal: the Mongols intended ‘to bring the whole world into subjection’ and Latin Europe must prepare.

The Ystoria Mongalorum compiles John's observations into a lengthy ethnographic report that covers a range of topics, including Mongol funeral rites, clothing construction, and a historiographic account of Chinggis Khan's unification of the Mongolian steppe. But because John concludes that ‘the Tartars never make peace except with those who submit to them’, the Ystoria most closely attends to Mongol military tactics. Throughout his minute depictions of Mongol warfare, John of Plano Carpini twice repeats how the Mongol military is divided ‘into groups under captains of a thousand, a hundred, and ten, and “darkness” – that is, ten thousand’. This description is, for the most part, accurate. While the Mongol army was indeed arranged by decimalization, John's description seems to confuse the Old Turkic and Mongolian word tumen (ten thousand) for duman (obscured by smoke). The error is a telling one. Each time John discusses the Mongol decimalization system, he is careful to translate tumen/duman twice; once as ‘darkness’, using the Latin tenebrae, and once as ‘ten thousand’. By providing two translations for the same word, the text insists that both meanings are significant. A ten-thousand strong troop unit, then, is always also a ‘darkness’. Given in the accusative case, tenebras further evokes the transitive verb tenebrare to which it is related.