frontier On his northern frontier Manuel expended considerable effort to preserve the conquests made by Basil II over one hundred years earlier and maintained, sometimes tenuously, ever since. Due to distraction from his neighbours on the Balkan frontier, Manuel was kept from his main objective, the subjugation of the Normans of Sicily. Relations had been good with the Serbs and Hungarians since 1129, so the Serb rebellion came as a shock. The Serbs of Rascia, being so induced by Roger II of Sicily, invaded Byzantine territory in 1149. Manuel forced the rebellious Serbs, and their leader, Uroš II, to vassalage (1150–1152). He then made repeated attacks upon the Hungarians with a view to annexing their territory along the Sava. In the wars of 1151–1153 and 1163–1168 Manuel led his troops into Hungary and a spectacular raid deep into enemy territory yielded substantial war booty. In 1167, Manuel sent 15,000 men under the command of Andronikos Kontostephanos against the Hungarians, scoring a decisive victory at the Battle of Sirmium and enabling the Empire to conclude a very advantageous peace with the Kingdom of Hungary by which Syrmia, Bosnia and Dalmatia were ceded. By 1168 nearly the whole of the eastern Adriatic coast lay in Manuel's hands. Efforts were also made towards a diplomatic annexation of Hungary. The Hungarian heir Béla, younger brother of the Hungarian king Stephen III, was sent to Constantinople to be educated in the emperor's court. Manuel intended the youth to marry his daughter, Maria, and to make him his heir, thus securing the union of Hungary with the Empire. At court Béla assumed the name Alexius and received the title of despot, which had previously been applied only to the emperor himself. However, two unforeseen dynastic events drastically altered the situation. In 1169, Manuel's young wife gave birth to a son, thus depriving Béla of his status as heir of the Byzantine throne (although Manuel would not renounce the Croatian lands he had taken from Hungary). Then, in 1172, Stephen died childless, and Béla went home to take his throne. Before leaving Constantinople, he swore a solemn oath to Manuel that he would always "keep in mind the interests of the emperor and of the Romans". Béla III kept his word: as long as Manuel lived, he made no attempt to retrieve his Croatian inheritance, which he only afterwards reincorporated into Hungary. Relations with Kievan Rus' (Russia)