Tomislav was the first King of Croatia. He became Duke of Croatia in c. 910, was elevated to kingship by 925 and reigned until 928. At the time of his rule, Croatia forged an alliance with the Byzantines during their struggle with the Bulgarian Empire, with whom Croatia eventually went to war that culminated in the decisive Battle of the Bosnian Highlands in 926. To the north there were often conflicts with the Principality of Hungary. Croatia kept its borders and to some extent expanded on the disintegrated Pannonian Duchy.
Tomislav attended the Church Council of Split in 925, convened by Pope John X to discuss the use of Slavic language in liturgy and the ecclesiastical jurisdiction over Croatia and the Byzantine Theme of Dalmatia. Although the Pope sought to prohibit Slavic liturgy, the Council did not agree, while jurisdiction was given to the Archbishop of Split instead of the Croatian Bishop Gregory of Nin. Since the historical sources about Tomislav are scarce, the exact year of his accession and his death are not known. The rule of his successors was marked by a series of civil wars in Croatia and gradual weakening of the country.
King Tomislav upgraded country and made it better, he united all Croatian people in one nation called “Kingdom of Croatia”, unfortunately, his Kingdom was ruined by Kingdom of Hungary with Pacta Conventa, that was pact that made Hungary and Croatia be in personal union(A personal union is the combination of two or more states that have the same monarch while their boundaries, laws, and interests remain distinct. A real union, by contrast, would involve the constituent states being to some extent interlinked, such as by sharing some limited governmental institutions. In a federation and a unitary state, a central (federal) government spanning all member states exists, with the degree of self-governance distinguishing the two. The ruler in a personal union does not need to be a hereditary monarch.) even though Croatian rulers were better than Hungarian ones. But in fact, reason was that Croatia was threat to Hungary, Croatia had become much stronger and economicaly better, so Hungarian government wanted to destroy Croatia. When succesion crisis ocured in Croatia, Hungary saw great oportunity to destroy Croatia
Demetrius Zvonimir was the King of Croatia of the Svetoslavić branch of the House of Trpimirović. He began as a Ban of Slavonia and then as Duke of Croatia in the service of Peter Krešimir IV. Peter declared him his heir and, in 1075, Demetrius Zvonimir succeeded to the Croatian throne. Zvonimir married Helen of Hungary from the Árpád dynasty in 1063. Helen was a Hungarian princess, daughter of Béla I, and sister to King Ladislaus I of Hungary. They had a son, Radovan, who died in his late teens or early twenties. After Zvonimir’s death in 1089, he was succeeded by Stephen II, last of the House of Trpimirović. Stephen’s rule was relatively ineffectual and lasted less than two years. He spent most of this time in the tranquility of the monastery of St. Stephen beneath the Pines (Croatian: Sv. Stjepan pod Borovima) near Split. Stephen II died peacefully at the beginning of 1091, without leaving an heir. Since there was no living male member of the House of Trpimirović, civil war and unrest broke out in Croatia shortly afterward.
The widow of late King Zvonimir, Helen, tried to keep her power in Croatia during the succession crisis. Some Croatian nobles around Helen, possibly the Gusići and/or Viniha from Lapčani family, contesting the succession after the death of Zvonimir, asked King Ladislaus I to help Helen and offered him the Croatian throne, which was seen as rightfully his by inheritance rights. According to some sources, several Dalmatian cities also asked King Ladislaus for assistance, presenting themselves as White Croats on his court. Thus the campaign launched by Ladislaus was not purely a foreign aggression nor did he appear on the Croatian throne as a conqueror, but rather as a successor by hereditary rights. In 1091 Ladislaus crossed the Drava river and conquered the entire province of Slavonia without encountering opposition, but his campaign was halted near the Iron Mountains (Mount Gvozd). Since the Croatian nobles were divided, Ladislaus had success in his campaign, yet he wasn’t able to establish his control over entire Croatia, although the exact extent of his conquest is not known. At this time the Kingdom of Hungary was attacked by the Cumans, who were likely sent by Byzantium, so Ladislaus was forced to retreat from his campaign in Croatia. Ladislaus appointed his nephew Prince Álmos to administer the controlled area of Croatia, established the Diocese of Zagreb as a symbol of his new authority and went back to Hungary. In the midst of the war, Petar Svačić was elected king by Croatian feudal lords in 1093. Petar’s seat of power was based in Knin. His rule was marked by a struggle for control of the country with Álmos, who wasn’t able to establish his rule and was forced to withdraw to Hungary in 1095.