The war broke out when Géza II of Hungary provided military assistance to the Serbs of Rascia (Raška), who rebelled against Byzantine suzerainty.

In the 1130s, Byzantine interests regarding Hungary were limited to maintaining the status quo from the 1129 treaty and to securing the empire's northernmost frontier along the Danube.

His blindness prevented him from administering his kingdom without assistance so he put his trust in his wife Queen Helena and her Serbian relatives, especially her brother Beloš.

The remains of Béla's father, former Hungarian pretender Prince Álmos, were returned from the Byzantine Empire to Hungary in 1137, supporting the existence of a permanent connection between the realms.

The pretender Boris successfully approached Conrad III of Germany to seek his assistance against Géza at the end of 1145, which worsened Hungary's relationship with the Holy Roman Empire.

[10][11] The pretender Boris attempted to take advantage of Conrad III's decision to lead the Second Crusade to the Holy Land through Hungary.

[12] Taking advantage of Conrad's crusade, which captured Manuel's attention, the Normans invaded; in 1147, they seized Corfu, and plundered Thebes and Corinth.

The following year, Manuel was distracted by a Cuman attack in the Balkans; he enlisted the alliance of Conrad III of Germany and the help of the Venetians, who quickly defeated Roger II of Sicily with their powerful fleet.

Iziaslav pursued an anti-Byzantine policy and expelled the metropolite from Kiev to end the ecclesiastical suzerainty of the Patriarch of Constantinople over the Kievan Rus'.

Bakchinos [Bágyon] a man of heroic stature and brawny arms who struck a blow that shattered the iron screen dependent from the helmet that protected the emperor's face and eyes.

The emperor, in turn, severed Bakchinos's arm with his sword, rendering him helpless, and took him captive.When the Serbs of Rascia rebelled against the Byzantine Empire in 1149, Manuel sent a retaliation force to crush the rebellion.

The emperor's troops pillaged the region, devastating Serbian towns and abducting inhabitants but Manuel was unable to capture Uroš and his courtiers.

[citation needed] The Hypatian Codex says Géza referred to his war against Manuel when excusing himself for refusing to send reinforcements to Iziaslav II, whom Yuri Dolgorukiy, Prince of Suzdal, expelled from Kiev in August 1149.

[18] Historians who say Hungary did not participation in the Serbian revolt argue the monarch referred to his membership in the Norman-led coalition against the Byzantine Empire, which he considered his enemy.

He resented the assistance they had given the Serbs, and he decided to take advantage of the absence...the Hungarian king [Geza II]... warring against the neighboring Rhos [Rus'].

Crossing the Sava River, he burst in upon Frangochorion [Syrmia] (this is not the least part of Hungary but a sufficiently populated one, situated between the Istros [Danube] and Sava rivers, in which a mighty fortress called Zevgminon [Zemun] had been built) and ravaged the land.According to John Kinnamos, the Byzantine army returned to Constantinople after the subjugation of the Serbs and then launched a retaliatory expedition against Hungary.

Ferenc Makk, based on a speech by Michael of Anchialus from 1155, said the conflict escalated into a war between the empire and Hungary in late 1150 or early 1151, which accords with all Byzantine, Russian and German chronicles.

A part of the army, under the leadership of Theodore Vatatzes, continued to besiege the fort and most of the imperial troops led by Manuel plundered and devastated the surrounding rich province of Syrmia.

[27][26] ...the Romans had constructed as many light boats as possible out of available materials and dragged them to the river, the Hungarians' king [Géza II]... lest being defeated a second time he imperil his realm; he proceeded to negotiations.

Sending envoys, he requested that Hungary should not be punitively deprived of more than ten thousand persons, but should regain the remaining throng of captives.

Thus he declared he would remain friendly to the Romans throughout his life... After peace had been concluded on these terms, the Romans' army started back from there [bank of the Danube].According to Ferenc Makk, Manuel entrusted Boris to launch an invasion against Hungary because he intended to warn Géza II, as emperor, he could easily realize Boris's claims to the Hungarian throne if he wanted.

Believing that he had secured the northern frontier of his realm by pacifying the Serbs and the Hungarians, Manuel returned his original plan to invade Sicily.

He sent a letter to Conrad III to re-join his cause, but the German monarch died suddenly in February 1152 and the succession of Frederick Barbarossa meant a turning point in the Byzantine–German relations in the longer term.

[35] ...the king of Hungary [Géza II] had assembled forces of Czechs and Saxons and many other nations, and settled down for a siege of Branitshevo [Braničevo]; [...] Hearing of this, the emperor [Manuel] was stunned by the report and wondered at the Hungarians' faithlessness, why for no reason they should disregard what had been lately pledged by them.

Others saved themselves by flight, among whom was the commander Basil.But this emperor [Manuel] once again declared war against the Paiones, who are also called Huns [Hungarians]... an embassy arrived from the Hungarians with peace proposals, and so, altering his course, he marched against the satrap of the Serbs [Uroš II], causing great consternation, and convinced him ...that he should renounce his treaty with the Hungarians.Despite the treaty, the relationship between Hungary and Byzantium remained tense and hostile.

Andronikos Komnenos' plot was discovered and he was captured, and Géza invaded the Byzantine Empire and laid siege to Braničevo in late 1154.

Géza initiated peace negotiations, and agreed to return the booty and Byzantine prisoners of war he had captured the previous year.

Economic difficulties arose due to Galician and Byzantine military actions, prompting Géza II to abandon an active foreign policy, including the support of Serbs.

[42] In the same year as the peace treaty, a Byzantine army expelled Géza's ally Desa, from Serbia and restored Uroš II, who had promised he would not enter into an alliance with Hungary.

The civil war was followed by a large-scale Byzantine invasion of Hungary until 1167, when Manuel seized significant territories in Croatia, Dalmatia, Sirmium and Bosnia.