Coastal Croatia or Dalmatian Croatia is the name for a duchy that was located in the area of the former Roman province of Dalmatia from the beginning of the 9th century, and included the area from the mountain Velebit in the north to the river Cetina in the south. It stretched inland along the rivers Neretva, Vrbas, Una and Kupa (today’s western and central Bosnia).
Croats in Dalmatia are mentioned after the Slavs came to this area and after the wars fought by Charlemagne. They founded Coastal Croatia, prevailing over other Slavs in the area, and over time, the name Croat was accepted by other Slavs in Croatia. In the time of Charlemagne, Croatia was a Frankish vassal country, and this continued later in relation to Eastern Francia.
In addition to coastal Croatia defined in this way, there are several other Slavic states, which together with Coastal Croatia are sometimes called “sclaviniae”. There was primarily Pagania or Neretva region, which stretched from the Cetina to the Neretva, and it also included the islands of Brač, Hvar, Korčula, Mljet, Vis and Lastovo. Further south were Zahumlje (between the Neretva and Dubrovnik), Travunja (from Dubrovnik to the Bay of Kotor) and Duklja (modern Montenegro). Inland, around the source of the river Bosna was Bosnia, and north of the Kupa was Pannonian Croatia.
From the 9th century, poorly organized territorial communities governed by clan chiefs formed the core of the Croatian state under the joint rule of dukes in the Cetina, Krka and Zrmanja river basins with centers in Biograd, Nin, Solinsko polje and Knin. The coastal cities of Zadar, Trogir, Split, Dubrovnik and Kotor, together with the islands, remained under Byzantine rule even after the 9th century. They were managed by the imperial governor in Zadar. This was the result of the Peace of Aachen concluded on January 12, 812, by which the Franks shared power in the Adriatic with Byzantium so that Dalmatian towns and islands belonged to Byzantium and the Croatian mainland to the Frankish area of interest.
After the death of Charlemagne, his son Louis the Pious came to power (814-840). In 814, together with duke Ljudevit of Pannonia, duke Borna of Coastal Croatia went to Paderborn to pay homage to the new emperor when he ascended the Frankish throne. The Friulian mark was ruled at that time by Margrave Kadolah, who soon came into conflict with the Pannonian Slavs and duke Ljudevit, and in 818 war broke out on the eastern borders of Francia. In these battles Borna will take the Frankish side, thus gaining for himself a strong force that will be able to defend him against the Byzantines. After the first unsuccessful campaign against Ljudevit, Kadolah died, and was succeeded by Margrave Balderic, who in 819 began a new campaign against the Slavs in Pannonia. Borna also marched from the south with his forces, now named in the “Frankish Chronicles” as the „dux of Dalmatia“ (duke of Dalmatia). However, near the river Kupa, a significant part of his forces sided with the Pannonian Croats. The following year, Borna first sended a delegation, and then personally went to the emperor to deliberate on future steps against Ljudevit.
The war in which Ljudevit was eventually defeated went to the advantage of the Coastal duke Borna, who consolidated his power in the area of Gacka, and as a reward for his loyalty to the Franks, he also received Liburnia under his rule. In this way, the real central authority over Coastal Croatia was established from the mouth of the Raša river in Istria, all the way to the Cetina. The „Theme of Dalmatia“ remained under Byzantine rule. Borna died in 821, and the “Frankish Chronicles”, on the occasion of his death, called him „dux Dalmatiae atque Liburniae“ („duke of Dalmatia and Liburnia“). Archaeological finds from that period testify to a considerable Frankish influence (numerous specimens of Frankish jewelry and weapons, especially swords, have been found).
Borna was succeeded as duke by his nephew (or grandson) Vladislav (821-835). In his time, tensions began with the Venetians and they would last for centuries. In February 821, Emperor Louis confirmed Vladislav as the duke of Dalmatia and Liburnia at the council in Aachen. His successor, Prince Mislav (835-845), had been at war with the Venetians since the very beginning of his reign. Not all the details of this conflict are known, but it is known that in 839 the doge Petar Tradonic set sail for Croatia, but in the end he was forced to make peace. In the same year, he made peace with the great prefect of Neretva, Družak. The following year, Neretva’s great prefect Deodat defeated the Venetians at sea, and they returned to Venice defeated. Prince Mislav, like his predecessors, did not rule from Nin, but from Klis. He was also the first known Croatian ruler to start giving gifts to the church in Split, giving it the parts of his Klis estate. Although Split, like other cities of „Theme Dalmatia“, belonged to the Byzantine government, it is obvious that the ties of these cities with the hinterland were becoming stronger at this time. Since 840, Saracen pirates have been a new problem in the Adriatic, attacking merchant ships and coastal towns. They were a common problem for Byzantium, Croats and Venice …