The Byzantine Empire was able to take advantage of the turmoil to expand its political influence and commercial relationships, first with the Khazars and later with the Rus' and other steppe groups. The Byzantines established the Theme of Cherson, formally known as Klimata, in the Crimea in the 830s to defend against raids by the Rus' and to protect vital grain shipments supplying Constantinople. Cherson also served as a key diplomatic link with the Khazars and others on the steppe, and it became the centre of Black Sea commerce. The Byzantines also helped the Khazars build a fortress at Sarkel on the Don river to protect their northwest frontier against incursions by the Turkic migrants and the Rus', and to control caravan trade routes and the portage between the Don and Volga rivers.