The siege of Constantinople in 860 was the only major military expedition of the Rus' people (Medieval Greek: Ῥῶς) 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 favour of the Khazars.

It is known from Byzantine sources that the Rus' caught Constantinople unprepared; preoccupied by the ongoing Arab–Byzantine wars, the empire was unable, at least initially, to make an effective response to the attack.

In April or May, both sides exchanged captives, and the hostilities briefly ceased; however, in the beginning of June, Emperor Michael III left Constantinople for Asia Minor to invade the Abbasid Caliphate.

[5] On June 18, 860,[a] at sunset, a fleet of about 200 Rus' vessels[b] sailed into the Bosporus and started pillaging the suburbs of Constantinople (Old East Slavic: Tsarigrad, Old Norse: Miklagarðr).

[6] Having devastated the suburbs, the Rus' passed into the Sea of Marmara and fell upon the Isles of the Princes, where the former patriarch Ignatius of Constantinople was living in exile.

[citation needed] In the 9th century, a legend sprang up to the effect that an ancient column at the Forum of Taurus had an inscription predicting that Constantinople would be conquered by the Rus.

On the other hand, Pope Nicholas I, in a letter sent to Michael III on September 28, 865, mentions that the suburbs of the imperial capital were recently raided by the pagans who were allowed to retreat without any punishment.

[11] The Venetian Chronicle of John the Deacon reports that the Normanorum gentes, having devastated the suburbanum of Constantinople, returned to their own lands in triumph ("et sic praedicta gens cum triumpho ad propriam regressa est").

[12] It appears that the victory of Michael III over the Rus' was invented by the Byzantine historians in the mid-9th century or later and became generally accepted in the Slavic chronicles influenced by them.