The Cumans first encountered the Rus' in 1055, when they advanced towards the Rus' Pereyaslavl principality, but Prince Vsevolod reached an agreement with them thus avoiding a military confrontation. In 1061, however, the Cumans, under the chieftain Sokal, invaded and devastated the Pereyaslavl principality; this began a war that would go on for 175 years. In 1068 at the Battle of the Alta River, the Cumans defeated the armies of the three sons of Yaroslav the Wise, Grand Prince Iziaslav I of Kiev, Prince Sviatoslav of Chernigov, and Prince Vsevolod of Pereyaslavl. After the Cuman victory, they repeatedly invaded Kievan Rus', devastating the land and taking captives, who became either their slaves or were sold at markets in the south. The most vulnerable regions were the Principality of Pereyaslavl, the Principality of Novgorod-Seversk and the Principality of Chernigov.