In 1162, the death of King Géza II (1141–62) presented the opportunity for Manuel I Komnenos (1143–80) to interfere in his neighbor’s realm. After a failed attempt to install an uncle of the reigning monarch, King Stephen III (1162–73), on the throne, the emperor reached a compromise whereby Géza’s youngest son Béla would live at the court in Constantinople and succeed Stephen as king. Béla married one of Manuel’s daughters, solidifying a Byzantine dynastic alliance. But Stephen continued to resist Byzantium in the Balkans, allying with the Holy Roman Empire under Frederick I Barbarossa (1155–90), Serbia, and the Russian principalities of Gallicia and Kiev. In violation of the treaty, Stephen designated his own son as his successor. In 1164, Stephen III and Duke Vladislav II of Bohemia marched to confront Manuel, who was stationed with his army on the Danube. Stephen agreed to cede to the empire the rich region of Syrmia, which was a family holding of Prince Béla, in exchange for the empire withdrawing its support for Stephen III’s uncle, also named Stephen, who had been fighting with Byzantine assistance to claim the throne. Later in the year, Stephen III seized Sirmium, a blatant act of war against the empire.
Manuel dislodged Frederick I Barbarossa from his Hungarian alliance, and pulled onto his side the Russian principality of Kiev, as well as Venice. Stephen’s forces busied themselves with the siege of Zeugminon (part of modern Belgrade, Serbia), which they seized by April 1165. Manuel led his forces northward in June 1165 and laid siege to Zeugminon. Manuel’s troops stormed the city on their third attempt and plundered the place mercilessly. In the meantime, Manuel’s general John Doukas had cut through Serbia and subdued the coastal cities and fortresses of Dalmatia, which Stephen III had also ceded as part of Béla’s holdings. In 1166 the Hungarians defeated Byzantine forces in Dalmatia and at Sirmium.
Manuel responded with the dispatch of his nephew, Andronikos Kontostephanos at the head of a strong Roman army, about one-third of which were mercenaries or allied foreigners. Roman scouts captured a Hungarian who revealed that the enemy force numbered 15,000 knights, bowmen, and light infantry. The Byzantine army was probably about equal in numerical strength. Kontostephanos drew up his marching order with Cuman and Turkish horse archers and a handful of western knights in the vanguard. Behind came three divisions of Byzantine regular cavalry and kataphraktoi, followed by units of allied Turkish and western mercenary cavalry. The last line comprised a mixed formation of Roman infantry and archers alongside a battalion of armored Turks, presumably also infantry.
Dénes, count of Bács, commanded the combined Hungarian-German force. Dénes drew up his mailed knights in the front, with infantry support to the rear. The historian Choniates noted that the Hungarian battle line was drawn up in a single, dense mass, in the shape of a tower; the cavalry fronted this deep formation. The Hungarian lancers presented an awesome sight—their horses wore frontlets and breastplates (these must have been padded or mail, since plate horse armor was uncommon in Europe prior to 1250) and carried riders mailed from head to foot. In short the Hungarian forces featured the best of modern western arms and equipment. They faced a lighter Byzantine force arrayed with the Turk and Cuman horse archers in the front of the formation. Behind, Andronikos divided his army into three divisions. On the left he stationed the regular Roman cavalry. In the center stood Andronikos, commanding elements of the Varangian Guard, Hetaireia imperial guard cavalry, Serbians, probably mailed cavalry, and Italian mercenary knights. The Roman right consisted of the third element of the line of march, with German mercenary knights and Turkish cavalry and Roman kataphraktoi cavalry. Behind the right and left wings of the army Andronikos stationed supporting troops, which presumably were mainly regular cavalry and infantry flank guards and outflankers who could also support the wings when pressured. That two of these supporting battalions were cavalry seems to be indicated by how the battle unfolded.
Andronikos opened the battle by sending ahead the Turk and Cuman horse archers and presumably the light infantry as well. They were instructed to send an arrow storm into the Hungarian cavalry and thus break up the formation. In the face of a Hungarian charge Andronikos instructed them to fan out to left and right and thus sweep to the side of the Byzantine force. The Byzantine left broke in the face of the Hungarian charge and fled toward the river Sava, but two battalions stood fast—these were likely the flank guards stationed behind the left wing. Dénes led a general charge into the Byzantine center, hoping to kill Andronikos; those in the center of the Roman formation sustained the heavy cavalry charge. The Byzantine right attacked the flank of the Hungarian cavalry formation, Andronikos’s men in the center of the line drew their iron maces and pressed forward for close combat, and the “routed” Byzantine left that had feigned flight returned to strike the Hungarian right flank. This envelopment broke the Hungarians, and thousands perished or were captured in the ensuing rout. Kinnamos reported that 2,000 cuirasses were taken from the dead, and countless shields, helmets, and swords came into Roman hands from the great number of fallen. The Battle of Sirmium was the greatest victory of Manuel’s reign; it demonstrated that tactical skill and great discipline were still to be found in the armies of the Komnenoi, as were commanders who were able to conceive and execute complicated battlefield maneuvers. As a result of Sirmium, Hungary became a client, and upon the death of Stephen III in 1172 Manuel easily installed his protégé Béla on the Hungarian throne, which remained at peace with the empire until 1180.
The campaigns of Manuel against Hungary that culminated in the Battle of Sirmium demonstrate that, when properly led, the Byzantine army remained the finest in eastern Europe, capable of defeating heavily armed and armored western knights. But these actions also show that the strategic situation of Byzantium had deteriorated significantly—with the coalescence of larger, more organized, and economically vibrant states on all sides, the empire faced extreme challenges to its territorial integrity. While Belisarios’s decisive victory over the Vandals a half millennium in the past had brought Africa under imperial control and established a peace that was largely maintained for a century, the “decisive” victory of Manuel at Sirmium delivered only twenty years of peace. In light of the capabilities of his enemies, it is small wonder that Manuel generally preferred attritive campaigns and small-war actions that wore down his foes and made enemy aggression too costly for them, rather than risking his limited forces in all-or-nothing engagements on the battlefield. In this sense, his failures are more telling than his numerous minor successes, since the emperor removed neither Sicily nor Hungary nor the Seljuks from their menacing positions along the frontiers. Instead, Manuel had to settle for a largely defensive posture in the territory he inherited from his father John.