This article analyses the importance of communication by letters during the initial months of the 1241 Mongol Invasion of Europe (ca. March–July 1241). It focuses especially on the ten letters found in the Ottobeuren collection (Innsbruck, Universitäts- und Landesbibliothek Tirol, Cod. 187, 1v–8v). Through a close reading of the collection and its visualization in the form of a network graph, this article reconstructs the transmission history of the Ottobeuren letters, including the report of Brother Julian, and shows the manner in which the collection was arranged by the compiler to give a pro-Hohenstaufen account of the invasion. The final section contextualizes the Ottobeuren letters as part of a wider correspondence network from these months, and offers a reappraisal of the importance of written communication in the actions of imperial princes involved in planning the defence of Germany and Bohemia from the Mongols.