The Sultan Kilidj Arslan realized that the isolation of the empire was the Seljuqs' opportunity. Frederick Barbarossa urged him to attack Byzantium, doubtless in the hope that this would lessen Turkish pressure on the crusader states. In 1175, after many years of virtual peace with the empire, Kilidj Arslan invaded Byzantine territory in Asia Minor. Manuel prepared for the major conflict that he had so long managed to avoid, and in 1176 he led what was virtually the total fighting force of the empire into Asia Minor. He hoped to expel the Turks from western Asia Minor for good, and his goal was the Seljuq capital of Konya. As his huge army, encumbered with baggage and siege engines, climbed from the Meander valley toward the Sultan Dagh, it had to traverse a pass, at the end of which stood an abandoned Byzantine fortress known as Myriokephalon. The Turkish army was stationed on the hills overlooking the pass, clearly visible to the Byzantines. There was some debate whether taking the Byzantine forces through such a defile in full view of the enemy was prudent. The hotheads prevailed, they posted flank guards, and pressed the army into the pass, brushing aside the Turks who tried to resist them. When they were well into the defile the enemy moved round them through the hills and charged into their flank and rear. For a time the Byzantines held on, pressed close together into a dense mass. Then Manuel, whether because he hoped to outflank the Turks or because his nerve failed, rode back out of the pass, followed by most of his army. They found themselves entangled with the baggage train, which blocked the narrow road. There the Turks were able to cut them down without mercy until nightfall. Finally Manuel accepted the sultan's offer of terms and withdrew with the remnants of his army. Kilidj Arslan may not have realized the extent of his victory - in any case his interest lay in the east and south, and Byzantium was of secondary importance. However, Manuel knew what he had lost, and himself compared his defeat at Myriokephalon with that of Romanos Diogenes at Mantzikert a century earlier. The formidable army built up by the sacrifices of the Byzantines had been annihilated. For the time being the empire was without military protection and surrounded by enemies, and Manuel's dream of making Byzantium once again a superpower was over. The last few insignificant Byzantine garrisons were expelled from Italy. The Hungarians began probing the northern frontiers, and the Serbs tried to shake off Byzantine control. There was little that Manuel could do about it. He had overstretched the empire's resources. The wealth accumulated by his father's careful husbandry he had spent on costly contingents of mercenaries. He had created a military ruling class - many of whose members were the hated westerners - which was divided from the rest of the population. The army had swallowed up the resources of the state and had in the end proved unable to protect it.
Manuel survived for four more years after the disaster of Myriokephalon and died in 1180. He left as his successor his twelve-year-old son Alexios II, under the regency of his widow, Maria of Antioch. The regent was detested as a westerner and was forced to rely more and more on the support of westerners in Constantinople. Many members of the imperial family were hostile to her and eager to challenge Manuel's will. Apart from their anti-Latin sentiments and their personal ambitions, they believed that the empire needed experienced and resolute leadership, not that of a boy and a foreign woman. Collectively they had plenty of military and political experience, since all three Komnenian emperors had regularly appointed members of their family to key military and civil positions. Yet they lacked effective leadership - perhaps years of subordination to so strong a personality as Manuel had made them unable to exercise leadership - and they failed to agree among themselves. It was left to one who was far from Constantinople, and who was in a way the black sheep of the family, to seize the reins of power that had fallen from Manuel's dying hands.
The enemies of the empire, who had been hovering like vultures over a dying beast since the destruction of the Byzantine army at Myriokephalon, swooping down upon their prey on the news of Manuel's death. In 1181 King Béla of Hungary seized Dalmatia, a large part of Croatia, and the key fortress of Sirmium guarding the old Roman military road to Constantinople. Stephen Nemanja wrested Serbia again from Byzantine control. Most of the western half of the Balkan peninsula was thus lost to the empire. Two years later, in 1183, the Hungarians and Serbs launched a major invasion. Their army marched unopposed down the military highway, capturing and virtually demolishing the cities on the way. Belgrade, Brani…evo, NiÓ and Sofia fell. In the west the Normans made preparations that were more leisurely but even more menacing. Not until summer 1185 did their fleet cross the Adriatic. Soon Dyrrhachium had been taken by storm, and the ungarrisoned islands of Corfu, Kephallonia and Zakynthos were occupied without opposition. The Norman fleet then sailed around Greece and by August anchored off Thessalonica. After a nine-days= siege - during which the garrison and its commander displayed neither competence nor courage - the second city of the empire fell. Violence and rapine on a terrifying scale accompanied the entry of the Normans, and they put many citizens to death with brutal and random cruelty. The wealth of the city and the priceless treasures of its churches were a temptation that the soldiers could not resist, and the Normans were probably simply unable to cope with the problems arising from the size of the city and the numbers of captives. The man who began negotiations with the Norman leaders and succeeded in persuading them to establish some kind of order was the archbishop of the city, Eustathius, a former teacher in the Patriarchal School in Constantinople and a notable scholar. The eyewitness account of which he wrote of the siege and its aftermath reveals unsparingly the breakdown of the Byzantine administrative system, and indeed of the bonds of Byzantine society, as well as the increasingly arbitrary brutality of an army of occupation.