The Rus' warlords launched several wars against the Khazar Qağanate, and raided down to the Caspian sea. The Schechter Letter relates the story of a campaign against Khazaria by "HLGW" (recently identified as Oleg of Chernigov) around 941 in which Oleg was defeated by the Khazar general Pesakh. The Khazar alliance with the Byzantine empire began to collapse in the early 10th century. Byzantine and Khazar forces may have clashed in the Crimea, and by the 940s emperor Constantine VII Porphyrogenitus was speculating in "De Administrando Imperio" about ways in which the Khazars could be isolated and attacked. The Byzantines during the same period began to attempt alliances with the Pechenegs and the Rus', with varying degrees of success. Sviatoslav I finally succeeded in destroying Khazar imperial power in the 960s, in a circular sweep that overwhelmed Khazar fortresses like Sarkel and Tamatarkha, and reached as far as the Caucasian Kassogians/Circassians and then back to Kyiv. Sarkel fell in 965, with the capital city of Atil following, c. 968 or 969.