Mislav was succeeded around 845 by duke Trpimir, who continued to recognize the supreme Frankish rule of King Lothar (840-855), but established a strong state system in his area, and his court did not differ significantly from the courts of other European rulers. Thus, at his court, Trpimir had court priests, court chamberlains, and a permanent entourage in which, among others, the most eminent prefects were included. The Arab attacks on the Mediterranean significantly weakened Byzantium and Venice, which Trpimir knew how to use, and in 846 and 848 he successfully fought against Byzantium on land and at sea, defeating the Byzantine archon who ruled in Zadar. Between 854 and 860, he successfully defended the country from Bulgarian attacks, and finally defeated them in somewhere in eastern Bosnia. The Venetians, it seems, were so weak that they did not even try to attack Croatia during Trpimir’s rule, because no source speaks of such conflicts.
In a charter written in Latin (preserved in a transcript from 1568, and according to recent research dated to 840 at the latest), by which he confirms the gift of duke Mislav to the diocese of Split and gives other gifts, Trpimir is called “dux Croatorum iuvatus munere divino” („Croatian duke by the grace of God“), and the land which he controls calls “regnum Croatorum”. In Latin, it literally means “kingdom of the Croats”. It can simply mean the land of the Croats, which is the only possible interpretation here, because the kingdom cannot yet be spoken of. However, the fact that he is the ruler “by the grace of God” and not by the grace of the emperor cannot be ignored, which could point to his sovereign power over the Croats. From this charter we learn details about Trpimir’s court (even the names of the courtiers), as well as that he owned Klis.
In Trpimir’s time, the diocese of Nin was probably established, responsible for the entire area under the rule of the Croatian ruler. He invited Benedictines from their center in Montecassino near Rome, and founded the first Benedictine monastery in Croatia, in Rižinice near Solin with the church of St. Petra. Stone fragments with the inscription “pro duce Trepim…” were found there, which could mean that Trpimir began his rule as a ban. Numerous churches are being built and rebuilt elsewhere. Trpimir himself made a pilgrimage to northern Italy, where in the famous Cheddar Gospel, along with the names of other European rulers (mostly Germanic and Slavic), his name is found as “domno Tripimiro” (lord Trpimir), with the title then given to the highest dignitaries. At the Trpimir court, fleeing from the Frankish rule, the Benedictine monk Gottschalk took refuge, leaving a description of the Croatian court in his writings. He calls Trpimir “King of the Slavs” (rex Sclavorum), which again signifies his supreme authority and the fact that he was no one’s vassal. Gottschalk, moreover, calls the inhabitants of the cities of “Theme of Dalmatia” Latins, and divides the other inhabitants of Dalmatia (which he identifies with the ancient Roman province) into the Dalmatians and Sclavinians, which is important information about the ethnic composition of Trpimir’s country.
For Trpimir’s authorities in the part of Coastal Croatia between Zrmanja and Cetina, only prefects are mentioned as regional governors, but not bans, which speaks of a different way of managing that part of the country the vicinity of Gacka. The end of Trpimir’s rule is not entirely clear, as is the exact sequence of his successors. The ambiguities and uncertainties of Trpimir’s dating in „De administrando imperio“ contribute to this. Trpimir had three sons (Petar, Zdeslav and Muncimir), but it seems that he was succeeded by duke Domagoj (he was not related to Trpimir), by force after he had one of his sons killed (probably Petar).
In the time of duke Trpimir, Croatia was extremely powerful militarily (victories in the conflicts with Byzantium and the Bulgarians), and the strength and influence of the Croatian state were stronger than ever before. From Trpimir came the genealogical branch of later Croatian rulers (and kings) which died out only in 1091 when King Stephen I died. Due to all this (as well as the number of sources) in Croatian historiography Trpimir is considered probably the most important person from the times of Croatian duchy.