Pecheneg troops pillaged Syrmia (now in Serbia) in 1071. As the king and the duke suspected that the soldiers of the Byzantine garrison at Belgrade incited the marauders against Hungary, they decided to attack the fortress. The Hungarian army crossed the river Sava, although the Byzantines "blew sulphurous fires by means of machines" against their boats. The Hungarians took Belgrade after a siege of three months. However, the Byzantine commander, Niketas, surrendered the fortress to Duke Géza instead of the king; he knew that Solomon "was a hard man and that in all things he listened to the vile counsels of Count Vid, who was detestable in the eyes both of God and men", according to the Illuminated Chronicle.

Division of the war-booty caused a new conflict between Solomon and his cousin, because the king granted only a quarter of the booty to the duke, who claimed its third part. Thereafter the duke negotiated with the Byzantine Emperor's envoys and set all the Byzantine captives free without the king's consent. The conflict was further sharpened by Count Vid; the Illuminated Chronicle narrates how the count incited the young monarch against his cousins by saying that as "two sharp swords cannot be kept in the same scabbard", so the king and the duke "cannot reign together in the same kingdom".