Ögedei Khan ordered Batu Khan to conquer Rus' in 1235. The main force, headed by Jochi's sons, and their cousins, Möngke Khan and Güyük Khan, arrived at Ryazan in December 1237. Ryazan refused to surrender, and the Mongols sacked it and then stormed Suzdalia. Many Rus' armies were defeated; Grand Prince Yuri was killed on the Sit River (March 4, 1238). Major cities such as Vladimir, Torzhok, and Kozelsk were captured.