Despite the disaster of 1204, the long-standing ties between the Byzantine (Nicene) empire and the Rus' principalities were not destroyed. Moreover, these ties were strengthened as the political power of Nicaea increased. The influence of Nicaea on Rus' showed itself not only in the ecclesiastical sphere, but also in the foreign policy of the Russian princes. Unfortunately, hardly any Ancient Russian or Greek source that shed light on the history of Rus'-Nicene relations in the early 13th century has survived. The evidence of Western European sources in Latin allows us to fill this gap to some extent. Those sources describe the history of contacts between the Catholic and the Orthodox churches and the failed attempts to achieve a church union.