In accordance with a peace treaty between his elder brother, Stephen III, who succeeded their father in 1162, and the Byzantine Emperor Manuel I Komnenos, Béla moved to Constantinople in 1163.

Taking advantage of the internal conflicts in the Byzantine Empire after Emperor Manuel's death, Béla reoccupied Croatia, Dalmatia and Sirmium between 1180 and 1181.

[11] The exact date of Géza II's grant cannot be determined, but according to Makk, Béla seems to have received his duchy around 1161.

[14][15] The Byzantine Emperor, Manuel I Komnenos, supported the uncles' takeover, but Stephen III returned to Hungary and regained his crown by force in the middle of 1163.

He therefore intended to unite in marriage Béla, who was [Géza II]'s son after [Stephen III], to his own daughter Maria."

[22][23] Emperor Manuel led his armies against Stephen, stating that he arrived "not to wage war on the Hungarians but to recover his land for Béla",[24] according to Kinnamos.

[29] After the imperial army recaptured Zimony (now Zemun in Serbia), Béla persuaded the emperor to prohibit the execution of the Hungarian soldiers who were captured in the fortress.

[29] Emperor Manuel ceremoniously made his daughter and Béla-Alexios his heirs, and forced the Byzantine notables to swear an oath of fidelity to them in the autumn of 1165.

[34] In the spring of 1166, Béla-Alexios accompanied protostrator Alexios Axouch, who led a Byzantine army against Hungary in retaliation for a new Hungarian invasion of Syrmium.

[35] On 11 April 1166, although Béla-Alexios and his bride were related to each other, Emperor Manuel confirmed a decision of the Ecumenical Patriarch, which stated that marriages between kin to the seventh degree were void.

[35] A new war broke out between Hungary and the Byzantine Empire in 1167, because Béla-Alexios "claimed the kingdom"[37] of his brother, according to the contemporaneous Rahewin.

[43][44] In the charter of grant, Béla-Alexios styled himself "Lord A., Duke of Hungary, Dalmatia and Croatia", ignoring the title that the emperor had recently bestowed upon him.

[45][46] Arnold of Lübeck, who was staying in Esztergom, recorded a rumor suggesting that Stephen had been poisoned by Béla's supporters, but no other source verifies this hearsay.

[48][40] They demanded that "Béla be dispatched to them as king", because "the principle of justice looked toward him" after his brother's death,[49] according to Kinnamos.

[51] Béla was unanimously elected king by the "dignitaries of the Hungarian kingdom", according to a letter written by Pope Alexander III in 1179.

[53][54] According to a scholarly theory, Archbishop Lucas also feared that the influence of "schismatics" would increase under Béla's rule.

[63] Meanwhile, Béla sent reinforcements to Emperor Manuel to help him fight against the Seljuks, but their united forces suffered defeat in the Battle of Myriokephalon on 17 September 1176.

[71] According to an inscription on a brick found in Bulkeszi (now Maglić in Serbia), Béla sponsored the baptism of a German "guest settler" in that village.

[74] According to the Illuminated Chronicle, Béla "introduced the same form of addressing petitions as was customary in the Roman and imperial court",[75] which suggests that the Royal Chancery began functioning as a separate office during his reign.

[84] Andronikos Komnenos accused the mother of the young Byzantine Emperor, Alexios II, of inciting Béla—her brother-in-law—to ravage the region of Belgrade and Barancs (now Braničevo in Serbia) in May 1182, implying that Béla had by that time occupied Syrmium.

[85][86] Taking advantage of the emerging anarchy in the Byzantine Empire, Béla advanced as far as Niš and Serdica in the first half of 1183.

[94] Orio Mastropiero, Doge of Venice, laid siege to Zadar in 1187, but the Venetian fleet could not seize the well-fortified town.

[99][96] Béla mediated a peace treaty between Frederick I and Isaac II, whose mutual distrust had almost caused war between the German crusaders and the Byzantines.

[97] In 1191, Béla met his son-in-law, Isaac II, in Philippopolis (now Plovdiv in Bulgaria) and Syrmium, but the results of their negotiations remained unknown.

[106] In 1194, Béla appointed his eldest son, Emeric, who had already been crowned as the future king, to administer Croatia and Dalmatia.

[109][110] However, his campaign was cancelled, because Béla's son-in-law, Emperor Isaac II, was dethroned by Alexios III Angelos in April 1195.

[15] His "rule not only represented the apogee of the kingdom of the Árpádians, but also marked the end of an epoch", according to historian Pál Engel.

[15] His establishment of the Royal Chancery contributed to the "expansion of written records" in Hungary; the first charters issued by barons appeared in the 1190s.

[129][130] According to a contemporaneous list of Béla's revenues, his yearly income amounted to almost 170,000 marks (about 23 tonnes of pure silver).

[131][133][134] In about 1190, after a fire destroyed Esztergom, Béla invited French masons to rebuild the royal palace and the cathedral.