Dmitar Zvonimir was most likely a descendant of the royal lineage descended from King Svetoslav Suronja, who fled to Venice during the civil war in Croatia in 1000, and later to Hungary.
It is assumed that Zvonimir ruled Sclavonia (Pannonian Croatia) during the times of Croatian king Petar Krešimir IV.which he managed with the help of the Hungarian house of Arpad with which he was related. His wife Jelena „The Beautyfull“ was the daughter of the Hungarian king Bela I. (1060-1063) and the sister of Ladislav I. (1077-1095). Sclavonia was conquered around 1027 by King Stephen I. of Hungary. Since the majority of the population in the area between the Sava and Drava rivers was of Slavic origin, it was ruled by Svetoslav’s descendants on behalf of the Hungarians.
Sometime after 1067, ban of Sclavonia, Zvonimir, with the help of the Hungarian king Solomon and duke Gejza, managed to expel the Duke of Istria and Carniola, Ulrik II. from the area of so-called “Marka Dalmatinska” (area of Kvarner, Croatian coast and part of Istria) who conquered this area around 1064. Since then, Zvonimir has ruled the area.
All this certainly greatly influenced King Petar Krešimir to take Zvonimir as his co-ruler and heir to the throne in 1070 (as such he has been mentioned in sources together with Petar Krešimir since that year), which is also interpreted in Croatian historiography as the reunification of Sclavonia with Croatia, which has thus become united and powerful again. Krešimir’s former heir to the throne, his nephew, duke Stjepan (eng. Stephen), was removed from that position and imprisoned in the monastery of “St. Stephen under the pines” in Split.
When the Normans captured (killed) King Petar Krešimir IV. in 1074, Zvonimir imposed himself as heir and became the new king. In September 1075 or 1076, Pope Gregory VII. he sent his legates, Abbot Gebizon and bishop Fulkon, with the main purpose of resolving the issue of recognizing the independence of the unified Croatian-Dalmatian state. After Dmitar Zvonimir swore that he would be a loyal subject of the pope, Gebizon crowned him on Sunday, October 8, 1075 or October 8, 1076 in Solin, in the new coronation basilica of St. Peter and Moses with the royal crown and handed him other signs of royal authority (scepter, sword and papal flag) and the Christian name Dmitar (lat. Demetrius). After the coronation, Dmitar Zvonimir holds the title of King of Croatia and Dalmatia (lat. rex Croatiae et Dalmatiae).
Thus, King Zvonimir, like some other European rulers, by securing the papal political doctrine, supporting the reform movement, and providing guarantees to church interests in Croatia and promising to take care of religious and family life, secured a political and defense alliance with the pope, state recognition of his state and its stable international position.
Among other things, the new king Dmitar Zvonimir promised to: help religious renewal and defend the Roman Church, give the pope an annual gift of money, then make sure that the Church is given first and tenths, prevent the sale of people, and protect the poor, widows and orphans . At the same time, Dmitar Zvonimir ceded to the Pope the monastery of St. Gregory in Vrana as a hospice for his deputies.
In 1079, Zvonimir was attacked by a nobleman from Istria, duke Vecelin, a subject of the Holy Roman Emperor Henry IV. Pope Gregory VII himself came to Zvonimir’s aid. and the conflict was successfully resolved. Somewhat later, Zvonimir went to war as an ally of the Norman duke Robert Guiscard against the Byzantine Empire and its ally Venice. In 1083, the Normans, in alliance with the Croats, defeated the Byzantine-Venetian fleet and captured the city of Durres, and in 1084. they severely defeated the Venetian fleet at Casop on the island of Corfu.
Dmitar Zvonimir strengthened the development of the economy and culture. He richly donated churches and monasteries, and in Biskupija near Knin he had a large three-nave basilica built. Of all his gifts, the most famous is the gift to the monastery of St. Lucija in Baška on the island of Krk, „Baška tablet“, whose text is completely preserved and represents one of the most important material finds related to Croatian early medieval rulers.
From his marriage to Jelena „The Beautyfull“, Zvonimir had a son Radovan and a daughter Klaudija. Since the only son and heir to the throne Radovan died before his father’s death (after 1083), the problem of inheriting the throne arose. After the death of Dmitar Zvonimir in 1089, the Croats brought Stephen II. to the throne, who was the son of King Petar Krešimir IV.’s brother Gojslav, but he also died very quickly in 1091, and Croatia fell into a long period of instability.