Ivan Asen sent envoys to Hungary before May 1240, most probably because he wanted to forge a defensive alliance against the Mongols. The Mongols' authority expanded as far as the Lower Danube after they captured Kiev on 6 December 1240. The Mongol expansion forced dozens of dispossessed Rus' princes and boyars to flee to Bulgaria. The Cumans who had settled in Hungary also fled to Bulgaria after their chieftain, Köten, was murdered in March 1241. According to a biography of the Mamluk sultan, Baibars, who was descended from a Cuman tribe, this tribe also sought asylum in Bulgaria after the Mongol invasion. The same source adds, that "A.n.s.khan, the king of Vlachia", who is associated with Ivan Asen by modern scholars, allowed the Cumans to settle in a valley, but he soon attacked and killed or enslaved them. Madgearu writes that Ivan Asen most probably attacked the Cumans because he wanted to prevent them from pillaging Bulgaria.