He was in conflict with dukes of Istria, while historical records "Annales Carinthiæ" and "Chronica Hungarorum" note he invaded Carinthia to aid Hungary in war during 1079/83, but this is disputed. Unlike Petar Krešimir IV, he was also an ally of the Normans, with whom he joined in wars against Byzantium. He married in 1063 Helen of Hungary, the daughter of King Bela I of the Hungarian Árpád dynasty, and the sister of the future King Ladislaus I. As King Zvonimir died in 1089 in unknown circumstances, with no direct heir to succeed him, Stjepan II ( 1089–1091) last of the main Trpimirović line came to the throne at an old age and reigned for two years.