 Boris and Oleg allied themselves with the Cumans and attacked Vsevolod on the Sozh River, defeating his army in a bloody battle and capturing Chernigov on 25 August 1078.Soon, Vsevolod and Izyaslav were able to muster a new army with the help of their sons and headed for Chernigov.Boris and Oleg had already left the city by the time Vsevolod and Izyaslav approached it, but the citizens of Chernigov closed the gates and prepared for the siege.