The siege of Constantinople of was the only major military expedition of the Rus' Khaganate recorded in Byzantine and Western European sources. The casus belli was the construction of the fortress Sarkel by Byzantine engineers, restricting the Rus' trade route along the Don River in favor of the Khazars. Accounts vary, with discrepancies between contemporary and later sources, and the outcome is unknown in detail. It is known from Byzantine sources that the Rus' caught Constantinople unprepared, while the empire was preoccupied by the ongoing Arab–Byzantine wars and was unable to respond effectively to the attack, certainly initially. After pillaging the suburbs of the Byzantine capital, the Rus' retreated for the day and continued their siege in the night after exhausting the Byzantine troops and causing disorganization. The Rus' turned back before attacking the city itself. The attack was the first encounter between the Rus' and Byzantines and led the Patriarch to send missionaries north to engage and attempt to convert the Rus' and the Slavs.