The Croatian duke Trpimir was not succeeded by any of his sons, but in 864 Domagoj came to power. He most likely had Trpimir’s eldest son Petar executed and thus became the prince of the whole of Coastal Croatia. His origins are unknown but it is probably from the Neretva area. The period of his reign was mostly marked by conflicts with the Venetians, in which he was very successful, so that the Venetians called him “pessimus dux Sclavorum” (“the worst duke of the Slavs”).
Already at the beginning of his reign he had to face the Venetian threat. Doge Orso Patrici (died 881) estimated that Croatia was weakened by the coup and decided to attack it in 865. After a short conflict, which interrupted the signed peace of 839, Domagoj managed to win the cessation of hostilities with diplomatic solutions.
The Saracens began to invade Byzantine coastal cities continuously during this period. Thus, in 866, Dubrovnik came under siege that lasted 15 months, and was eventually interrupted with the help of the Byzantine navy. Byzantium used the siege to re-establish its initiative in the Adriatic. Byzantine rule was slowly being restored in the coastal cities, but also in areas with a Slavic population („the sclavinias“), which enabled them to go along with the Franks to the siege of Bari, which was under Arab rule. Slavs from all Adriatic sklavini as also took part in the siege. It was at this time of joint reckoning with the Arabs that the relations of the two strongest Christian empires, the Byzantine and the Frankish, deteriorated. The conflict culminated in the Council of Constantinople in 870, when a fierce quarrel broke out between the papal envoys and the patriarch of Constantinople over ecclesiastical jurisdiction over Bulgaria and southern Italy. Byzantine Emperor Basil I (c. 812-886), disappointed by the inactivity of Ludwig II (c. 825-875), the successor to Frankish rule in Italy, withdrew his troops from the conflict with the Arabs and the burden of the struggle fell on the Franks. As it was not possible to capture Bari without the navy, Ludwig called on Domagoj for help, with whose help Bari was finally conquered in 871.
It seems that by participating in the successful siege of Bari, Domagoj gained enough self-confidence to continue the war at sea for his own interests. After Trpimir’s wars on land, which strengthened the continental borders of the principality, Domagoj focused on warfare at sea to secure the coast. Although the Arab danger still existed, Domagoj’s main enemies were the Venetians. The clashes began around 872 near the Istrian coast in which the Venetians suffered several heavy defeats.
Towards the end of Domagoj’s reign, around 874, there was a conspiracy against him. There is no concrete mention of the participants in the sources, but a large part of historians believe that they were Trpimir’s sons, to whom Byzantium provided support. However, the conspiracy was uncovered, and its participants were promptly executed. Domagoj’s purges were so cruel that even Pope John VIII. in a letter of 874 he amicably advises not to kill the rebels but to expel them. In the same letter, Pope adresses Domagoj as “dux Sclavorum” (“duke of the Slavs”), which speaks volumes about Domagoj’s reputation and power.
Domagoj’s death in 876 led to a two-year conflict over the inheritance of the Croatian duchy, which was briefly interrupted in 878 by the Byzantine installation of Trpimir’s son Zdeslav in power. As early as next year, in a new conspiracy, Zdeslav was killed, and the princely rule would be stabilized under duke Branimir. Duke Domagoj will be remembered as a warlike ruler who successfully fought against the Venetians and Arabs. At the same time, he established Coastal Croatia on the then political map of Europe, slowly resolving its Byzantine and Frankish influence and paving the way for its independence.