The Seljuk Sultan of Rûm Kayqubad II appealed to Berke, khan of the Golden Horde, to attack the Byzantine Empire in order to free his brother Kaykaus II. The Mongols crossed the frozen Danube river in the winter of 1263/1264. They defeated the armies of Michael VIII in the spring of 1264. While most of the defeated army fled, the Byzantine Emperor escaped with the assistance of Italian merchants. After that Thrace was plundered. Michael VIII was forced to release Kaykaus, and signed a treaty with Berke, in which he agreed to give one of his daughters, Euphrosyne Palaiogina, in marriage to Nogai. Berke ceded Crimea to Kaykaus as appanage and agreed that he would marry a Mongol woman. Michael also sent tribute to the Horde.