In the treaty, Géza and his brother Ladislaus acknowledged the rule of Solomon, who granted them their father's former duchy, which encompassed one-third of the Kingdom of Hungary.

[7] According to the traditional principle of seniority, Béla preserved his claim to succeed his brother even after Andrew's wife Anastasia of Kiev gave birth to Solomon in 1053.

[1][8] The Illuminated Chronicle narrates that the child Solomon "was anointed king with the consent of Duke Bela and his sons Geysa and Ladislaus",[9] which is the first reference to a public act by Géza.

[1] However, according to the contemporaneous text Annales Altahenses, Géza was absent from the meeting where Judith—the sister of the German monarch Henry IV—was engaged to the child Solomon in 1058.

Lampert of Hersfeld wrote that Géza persuaded his father to set free Count William of Weimar, one of the commanders of the German troops fighting on Andrew's side, who had been captured in a battle.

[19][21] According to the treaty, which was signed in Győr on 20 January 1064, Géza and his brothers accepted Solomon's rule and the king granted them their father's duchy.

[3][22] The king and his cousins celebrated Easter together in the cathedral of Pécs, where Duke Géza ceremoniously put a crown on Solomon's head.

[23] Being a newcomer and not yet established in his kingdom, King [Solomon] was afraid that [Géza] would perhaps attack him with a Polish army, and he therefore retired for a time with his forces and took up a safe station in the strongly fortified castle of [Moson].

Especially bishop Desiderius softened Duke [Géza]'s spirit with his gentle admonitions and sweet pleadings that he should peaceably restore the kingdom to [Solomon], even though he was the younger, and should himself assume the dukedom which his father had held before him.

At [Győr], on the feast day of SS Fabian and Sebastian the martyrs, King [Solomon] and Duke [Géza] made peace with each other before the Hungarian people.According to Ján Steinhübel and other Slovak historians, Géza only retained the administration of the region of Nyitra (present-day Nitra, Slovakia) and gave the eastern territories of their father's duchy, which were centered around Bihar (present-day Biharia, Romania), to his brother, Ladislaus.

[23] The historians Gyula Kristó and Ferenc Makk write that Géza seems to have married a German countess named Sophia around this time.

[29][30] The conflict between the king and his cousins was sharpened by Solomon's main advisor, Count Vid, who wanted to acquire the dukes' domains for himself.

[31] At a meeting in the Szekszárd Abbey, Count Vid persuaded the king to break the truce in order to unexpectedly attack Géza who was "hunting in Igfan Forest"[32] to the east of the river Tisza.

[30][31][33] A legend preserved in the Illuminated Chronicle mentions that before the battle, Ladislaus "saw in broad daylight a vision from heaven" of an angel placing a crown on Géza's head.

[34][35] Another legendary episode also predicted the dukes' triumph over the king: an "ermine of purest white" jumped from a thorny bush to Ladislaus's lance and then onto his chest.

[37] Géza and his allies won a decisive victory and forced the king to flee from the battlefield and to withdraw to Moson at the western frontier of Hungary.

[33][37] Géza "made" Kapuvár, Babót, Székesfehérvár and "other castles secure with garrisons of the bravest soldiers",[39] thus taking possession of almost the entire kingdom.

The patriarch, therefore, and the dukes, seduced by the gifts and possessed with love of gold, invented various false stories to induce the Emperor to turn back.

The patriarch pretended that he had a dream whose interpretation most plainly was that the Emperor's army would be wholly destroyed by the divine vengeance unless he returned with the utmost speed.

The dukes pretended likewise to be awestricken by divine warnings ...In early 1074, Géza had approached Pope Gregory VII to obtain international recognition of his rule.

[49] In this year he styled himself as "anointed king of the Hungarians by the grace of God" in the charter of the foundation of the Benedictine Abbey of Garamszentbenedek (present-day Hronský Beňadik, Slovakia).

[21] A grave discovered in the center of the medieval cathedral in August 2015 was identified as Géza's burial site by Zoltán Batizi, the leader of the excavations.