Stjepan I. (eng. Stephen) was the son of king Krešimir III. who died sometime around 1030 AD. During his reign, he tried to regain the lost power of the Croatian kingdom, which he largely succeeded in doing. Namely, he inherited a weak state that he wanted to wrest from Byzantine influence and regain the coastal cities occupied by Venice.
In his time, however, the political situation changed somewhat. The powerful Byzantine emperor Basil II. he died in 1025, which was already used by Krešimir III. who ceased to pay to Byzantines a monetary tax. Stjepan I. continued where his father left off but had to be wise because the Byzantine Empire and Venice were allies with similar interests.
In 1032. he sent Croatian ships to help the Byzantine navy in their war against the Arabs. With this act, Stjepan wanted to strengthen Croatian-Byzantine ties and at the same time prevent possible Byzantine aid to the Venetians in the eastern Adriatic. By the end of his reign, Stjepan I. managed to regain all Dalmatian cities except Zadar.
Also at that time, Croats and Serbs in Duklja, Travunja and Zahumlje were trying to free themselves from Byzantine rule under the leadership of Duklja’s prince Stjepan Vojislav. They succeeded in that, and in 1042. Stjepan Vojislav founded the independent state of Duklja. In the same year, Constantine IX. Monomach became the Byzantine emperor who at all costs wanted to regain the lost influence in this area. In order to preserve parts of Dalmatia, the emperor appointed the Croatian ban Stjepan Praska as the “imperial protospator” (honorary commander of the imperial military branch) who served Byzantine interests and sat in Zadar.
In 1046, king Petar “the Venetian” , the son of the former Venetian doge Otto Orseolo and the nephew of the Croatian king Stjepan I, had to flee Hungary. This situation dragged Stjepan I. into wars with the Hungarians. Venice took advantage of these opportunities and, under the leadership of the then doge Dominic Contarini, attacked Croatia and in 1050. took over Zadar. Other cities remained within the Croatian kingdom.
As a reflection of his independence, Stjepan I. founded a diocese in Knin in 1040, which reached all the way to the Drava river in the north. The bishop of Knin wore the title “Croatian bishop” (“episcopus Chroatensis”) and the first bishop in Knin from 1042. was bishop Marko.
Trade and economy flourished during the reign of Stjepan I. Also the aristocratic (patrician) citizenry was getting stronger in cities such as Zadar, Split, Biograd, Knin, but also other coastal cities. At that time cities in Sclavonia (Northern Croatia) also become to emerge. In that area, there was much more fertile and arable land than on the coast. The inhabitants were mostly engaged in agriculture, craft and trade.
We learn the most about the reign of King Stjepan I. from foreign sources and from the documents of his son King Peter Krešimir IV. Perhaps the most important of them is the one from 1066/1067. by which Petar Krešimir IV. confirms that the monastery of St. Krševan in Zadar owns a piece of land in Diklo because it gives us a broader overview of family relations within the ruling dynasty. In it Petar Krešimir IV. among other things, he lists his ancestors. He starts with Mihajlo Krešimir II. whome he calls “Chresimiri maioris”, and states that he had a son Držislav (“Dirzislai”), during whose time the strong bans Pribina and Godemir ruled. After Držislav, Svetoslav (“Suataslao”) is mentioned, who had brothers Krešimir (“Cresimiri”) and Gojslav (“Goyslaui”) and a son Stjepan, for whom Petar Krešimir IV. he says he is his father (“pater mei”).
We do not know exactly how long Stjepan I. lived, but he probably died around 1055, because in sources from 1058, Petar Krešimir IV. is mentioned as the Croatian king.