Its existence was characterized by various conflicts and periods of peace or alliance with the Bulgarians, Byzantines, Hungarians, and competition with Venice for control over the eastern Adriatic coast.

The goal of promoting the Croatian language in the religious service was initially introduced by the 10th century bishop Gregory of Nin, which resulted in a conflict with the Pope, later to be put down by him.

[7][8][9] The nature of the relationship varied through time, with Croatia retaining a large degree of internal autonomy overall, while the real power rested in the hands of the local nobility.

[16] Another often Hungarian synonym, as used by Ladislaus I of Hungary in his 1091 diplomatic letter, was Sclavoniam iam fere totam (acquisivi) ("Whole of S(c)lavonia").

First in a note preceding the text of the conclusions of the Council of Split in 925, where it is written that Tomislav is the "king" ruling "in the province of the Croats and in the Dalmatian regions" (in prouintia Croatorum et Dalmatiarum finibus Tamisclao rege),[20][21][22] while in the 12th canon of the Council conclusions the ruler of the Croats is called "king" (rex et proceres Chroatorum).

Although the exact geographical extent of Tomislav's kingdom is not fully known, Croatia probably covered most of Dalmatia, Pannonia, and northern and western Bosnia.

[29] According to the contemporary De Administrando Imperio, the Croatian army and navy at the time could have consisted of approximately 100,000 infantry units, 60,000 cavaliers, and 80 larger (sagina) and 100 smaller warships (condura),[30] but these numbers are generally taken as a considerable exaggeration.

[26] According to the palaeographic analysis of the original manuscript of De Administrando Imperio, the population of medieval Croatia was estimated at between 440,000 and 880,000 people, while the military force was most probably composed of 20,000–100,000 infantrymen and 3,000–24,000 horsemen organized into 60 allagions.

Local leaders, the župani, were replaced by the retainers of the king, who took land from the previous landowners, essentially creating a feudal system.

Tomislav was succeeded by Trpimir II (c. 928–935) and Krešimir I (c. 935–945), who each managed to maintain their power and keep good relations with both the Byzantine Empire and the Pope.

Helen died on 8 October 976 and was buried in that church, where a royal inscription on her sarcophagus was found that called her "Mother of the Kingdom".

[35][36] Michael Krešimir II was succeeded by his son Stephen Držislav (969–997), who established better relations with the Byzantine Empire and their Theme of Dalmatia.

[36] As soon as Stjepan Držislav had died in 997, his three sons, Svetoslav (997–1000), Krešimir III (1000–1030), and Gojslav (1000–1020), opened a violent contest for the throne, weakening the state and allowing the Venetians under Pietro II Orseolo and the Bulgarians under Samuil to encroach on the Croatian possessions along the Adriatic.

[39] He also allowed the Roman curia to become more involved in the religious affairs of Croatia, which consolidated his power but disrupted his rule over the Glagolitic clergy in parts of Istria after 1060.

The names of court titles in their vernacular form appear for the first time during his reign, such as vratar ("door-keeper") Jurina, postelnik ("chamberlain") and so on.

[40] The Roman Catholic Church reforms, which imposed a ban on the use of Slavonic liturgy and introduced Latin as obligatory, were confirmed by Pope Alexander II in 1063.

Zvonimir's kinghood is carved in stone on the Baška Tablet, preserved to this day as one of the oldest written Croatian texts, kept in the archæological museum in Zagreb.

[42] According to the Provinciale Vetus (published in late 12th century), in the Kingdom of Croatia and Dalmatia around 1075 existed 20 bishopric civitas, including civitas Salona (Solin-Split), Jadera (Zadar), Scandona (Skradin), Tragurium (Trogir), Belgradum (Biograd na Moru), Arbum (Rab), Absara (Osor), Vecla (Krk), Sissia (Sisak), Ragusium (Dubrovnik), Catara (Kotor), Stagnum (Ston), Mucrona (Muccur, Makar), Bosna (Visoko), Dulcinium (Ulcinj), Suacium (Svač), Antibarum (Bar), Delmenia (Omiš) and Nona (Nin).

[54] Since the Croatian nobles were divided, Ladislaus had some success in his campaign, yet he wasn't able to establish his control over the entirety of Croatia, although the exact extent of his conquest is not known.

[49] 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.

[56] Coloman assembled a large army to press his claim on the throne and in 1097 defeated King Petar's troops in the Battle of Gvozd Mountain, where the latter was killed.

In return, the 12 Croatian nobles that signed the agreement retained their lands and properties and were granted exemption from tax or tributes.

[57][58] Despite the fact that the Pacta Conventa is not an authentic document from 1102, there was almost certainly some kind of contract or agreement between the Croatian nobles and Coloman which regulated the relations in the same way.

Although these kinds of claims can also be found today, since the Croatian-Hungarian tensions are gone, it has generally been accepted that Coloman was crowned in Biograd as king.

[71] According to some historians, Croatia became part of Hungary in the late 11th and early 12th century,[72] yet the actual nature of the relationship is difficult to define.

[10] The alleged agreement called Pacta conventa (English: Agreed accords) or Qualiter (first word of the text) is today viewed as a 14th-century forgery by most modern Croatian historians.

[61] The official entering of Croatia into a personal union with Hungary, later becoming part of the Lands of the Crown of St. Stephen,[73] had several important consequences.

Coloman's successors continued to crown themselves as Kings of Croatia separately in Biograd na Moru until the time of Béla IV.

[74] In the 14th century a new term arose to describe the collection of de jure independent states under the rule of the Hungarian King: Archiregnum Hungaricum (Lands of the Crown of Saint Stephen).

In March 2024, Sabor proclaimed 2025 as "The year of marking the 1100th anniversary of the Kingdom of Croatia" (Croatian: Godina obilježavanja 1100. obljetnice hrvatskoga kraljevstva).