Rus' Khaganate
The siege of Constantinople of 860 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.