Tomislav of Croatia in the context of "Pope John X"

Croatia, officially the Republic of Croatia, is a country in Central and Southeast Europe, on the coast of the Adriatic Sea. It borders Slovenia to the northwest, Hungary to the northeast, Serbia to the east, Bosnia and Herzegovina and Montenegro to the southeast, and shares a maritime border with Italy to the west. Its capital and largest city, Zagreb, forms one of the country's primary subdivisions, with twenty counties. Other major urban centers include Split, Rijeka and Osijek. The country spans 56,594 square kilometres (21,851 square miles), and has a population of nearly 3.9 million.

The Croats arrived in modern-day Croatia, then part of Roman Illyria, in the late 6th century. In the 7th century, they organized the territory into two duchies. Croatia was first internationally recognized as independent on 7 June 879 during the reign of Duke Branimir. Tomislav became the first king by 925, elevating Croatia to the status of a kingdom. During the succession crisis after the Trpimirović dynasty ended, Croatia entered a personal union with Hungary in 1102. In 1527, faced with Ottoman conquest, the Croatian Parliament elected Ferdinand I of Austria to the Croatian throne. In October 1918, the State of Slovenes, Croats, and Serbs, independent from the Habsburg Empire, was proclaimed in Zagreb, and in December 1918, it merged into the Kingdom of Yugoslavia. Following the Axis invasion of Yugoslavia in April 1941, most of Croatia was incorporated into a Nazi-installed puppet state, the Independent State of Croatia. A resistance movement led to the creation of the Socialist Republic of Croatia, which after the war became a founding member and constituent of the Socialist Federal Republic of Yugoslavia. On 25 June 1991, Croatia declared independence, and the War of Independence was successfully fought over the next four years.

The Duchy of Croatia (Modern Croatian: Kneževina Hrvatska; also Duchy of the Croats, Modern Croatian: Kneževina Hrvata; Latin: Ducatus Chroatorum; Greek: Χρωβατία) was a medieval state that was established by White Croats who migrated into the area of the former Roman province of Dalmatia c. 7th century AD. Throughout its existence the Duchy had several seats – namely, Klis, Solin, Knin, Bijaći and Nin. It comprised the littoral – the coastal part of today's Croatia – except Istria, and included a large part of the mountainous hinterland as well.

The Croats settled in Dalmatia after defeating the Pannonian Avars, during the time of Byzantine emperor Heraclius I. The Duchy was in the center of competition between the Byzantine Empire and the Carolingian Empire for rule over the area. Croatian rivalry with Venice emerged in the first decades of the 9th century and would continue through the following centuries. Croatia also waged battles with the Bulgarian Empire (founded c. 681; Bulgar-Croatian relations improved greatly afterwards) and with the Arabs; it also sought to extend its control over important coastal city-states under the rule of Byzantium. Croatia experienced periods of vassalage to the Franks or to the Byzantines and of de facto independence until 879, when Duke Branimir was recognized as an independent ruler by Pope John VIII. The Duchy was ruled by the Trpimirović and Domagojević dynasties from 845 to 1091. Around 925, during the rule of Tomislav, Croatia became a kingdom.

The Trpimirović dynasty (Croatian: Trpimirovići) was a native Croatian dynasty that ruled in the Duchy and later the Kingdom of Croatia, with interruptions by the Domagojević dynasty from 845 until 1091. It was named after Trpimir I, the first member and founder. The most prominent rulers of the Trpimirović Dynasty include Tomislav (first king of Croatia), Petar Krešimir IV and Demetrius Zvonimir. The house gave four dukes, thirteen kings and a queen.

The Klis Fortress (Croatian: Tvrđava Klis; Italian: Fortezza di Clissa) is a medieval fortress situated above the village of Klis, near Split, Croatia. From its origin as a small stronghold built by the ancient Illyrian tribe Dalmatae, to a role as royal castle and seat of many Croatian kings, to its final development as a large fortress during the Ottoman wars in Europe, Klis Fortress has guarded the frontier, being lost and re-conquered several times throughout its 2,000-year history. Due to its location on a pass that separates the mountains Mosor and Kozjak, the fortress served as a major source of defense in Dalmatia, especially against the Ottoman Empire. It has been a crossroad between the Mediterranean Sea and the Balkans.

Since Duke Mislav of the Duchy of Croatia made Klis Fortress the seat of his throne in the middle of the 9th century, the fortress served as the seat of many Croatia's rulers. His successor, Duke Trpimir I, is significant for spreading Christianity in the Duchy of Croatia. He expanded the Klis Fortress, and in Rižinice [hr], in the valley under the fortress, he built a church and the first Benedictine monastery in Croatia. During the reign of the first Croatian king, Tomislav, Klis and Biograd na Moru were his chief residences.

In 926 a battle was fought in the Bosnian highlands between the armies of the Bulgarian Empire, under the rule of Bulgarian Tsar Simeon I, who at the time also fought a war with the Byzantine Empire, and the Kingdom of Croatia under Tomislav, the first king of the Croatian state. The battle is also known as the Battle of the Bosnian Highlands (Bulgarian: Битка при Босненските планини, Croatian: Bitka na Bosanskim visoravnima). It was fought in the Dinaric Alps of Eastern Bosnia near the rivers Bosna and Drina, the border area between the Kingdom of Croatia and the Bulgarian Empire.

Principal information on the battle is provided by the emperor Constantine VII of the Byzantine Empire in his work De Administrando Imperio ("On the Governance of the Empire") and in the collection of preserved historical writings called Theophanes Continuatus. Simeon's aim was to defeat the Byzantine Empire and conquer Constantinople. To achieve his aim, Simeon overran the eastern and central Balkans several times, occupied Serbia and finally attacked Croatia. The result of the battle was an overwhelming Croatian victory.