Béla settled in Poland and married Richeza (or Adelaide), daughter of Polish king Mieszko II Lambert.

Andrew assigned the administration of the so-called ducatus or "duchy", which encompassed around one-third of the territory of the Kingdom of Hungary, to Béla.

[2] The chronicles also refer to gossip claiming that the three brothers were their father's illegitimate sons, born to "a girl from the Tátony clan".

The former view is represented, for example, by the Polish historian Wincenty Swoboda,[6] and the latter by the Hungarian scholars Gyula Kristó and Ferenc Makk.

[15][16] According to the Hungarian chronicles, Béla participated in a Polish expedition against the pagan Pomeranians and defeated their duke "in single combat".

[14][17] The Illuminated Chronicle narrates that the Polish monarch "praised the boldness and strength of Duke Béla and bestowed on him all the Pomeranian tribute".

[21] According to Kristó and Makk, Béla was staying in Poland during this period; he may even have administered the kingdom in the name of his absent brother-in-law.

[17] On the other hand, the Polish historian Manteuffel writes that Béla and his two brothers, in contrast with the unanimous report of the Hungarian chronicles, arrived in Poland only with Casimir, after 1039.

[28][29] Béla's ducatus or "duchy" encompassed large territories along the eastern and northern borders, including the regions of Nyitra (Nitra, Slovakia) and Bihar (Biharia, Romania).

[33] Béla was one of the lords witnessing the deed of the foundation of the Tihany Abbey, a Benedictine monastery that his brother established in 1055.

[30] He invited Béla to his manor in Tiszavárkony, where the King offered his brother a seemingly free choice between a crown and a sword (which were the symbols of the royal and ducal power, respectively).

[36] Having been informed of his brother's secret plan by one of his own partisans in the royal court, Béla opted for the sword, but he departed for Poland after the meeting.

[37] The ensuing civil war ended with the victory of Béla, who defeated his brother in two successive battles fought at the river Tisza and at Moson.

[39] He ordered that "the wives and sons and all the property of all those who had followed" his nephew to Germany "should be protected and kept safe and sound",[41] which induced many of Solomon's partisans to reconcile themselves to Béla's rule and return to Hungary.

[42] The historian Nora Berend says that the latter measure "may have adversely affected Jewish activities", because Jews, who observed the Sabbath, could not work on Saturdays.

[43] Béla decided to discuss his innovations with the representatives of the freemen, and "sent heralds throughout all Hungary to summon two elders with gift of speech from each village to a royal council",[44] according to the Illuminated Chronicle.

[30] They demanded the restoration of paganism and the murder of clergymen, but Béla collected his army and suppressed their uprising within three days.

[48] In the summer of 1063, an assembly of the German princes decided to launch a military expedition against Hungary to restore young Solomon to the throne.

[48] Béla was planning to abdicate in favor of his nephew if the latter restored his former ducatus, but he was seriously injured when "his throne broke beneath him"[49] in his manor at Dömös.

[54] According to the historian Martin Dimnik, Béla also fathered a fifth daughter, Lanka, who was the wife of Prince Rostislav Vladimirovich of Tmutarakan.