During the reign of Tomislav (910-928), Croatia, along with Bulgaria, became the strongest state in the area between Germany and Constantinople. He organized a strong army and navy, defeated the Magyars and forced them across the Drava River. Since then this river has been the age-old border between these two peoples. Tomislav also annexed Sava Croatia to his state. This included the area between the Drava, Sava and Kupa Rivers, which would later be known as Slavonia, i.e. the land of the Slavs (Slaveni, Slovini). This was the first time that the two Croatian principalities were united. As an ally of Byzantium, Tomislav also defeated the Bulgarian army, so that he was granted the right to administer the coastal cities (Zadar, Split, Trogir, so-called Byzantine Dalmatia) and thus rounded off his state from the Adriatic Sea to the Drava River, and from the Rasa River in Istria to the Drina River.
At the height of his power Tomislav crowned himself as king (925). After this, at two church synods in Split he established the relationship between the Split, Latin and Nin, Croatian, diocese, and in this way strengthened internal relations in Croatia and resolved the dispute around the liturgical language between the Latinists and Croatian Glagolites. Through this Tomislav obtained the support of the pope, who was at that time an extremely important international power. After Tomislav several kings ruled Croatia, of which the following are the most notable: Drzislav, who received his crown from Constantinople as the king of Dalmatia and Croatia (969-997); Petar Kresimir IV, during whose reign Croatia achieved its greatest territorial extent (1058-1074); and Dmitar Zvonimir, who ruled at the time of Croatia's greatest economic progress (1074-1089). All of these kings bore the recognized title of king of Dalmatia and Croatia, administered the wealthy Dalmatian towns, ruled the sea routes in the Adriatic Sea and resisted both Venice and Constantinople.
During the reign of Dmitar Zvonimir a charter was engraved in stone, known as the Baska Tablet, which contains the king's name and title. This tablet has been preserved and it is the oldest artifact written in the Croatian language and the Glagolitic script. It is considered that this was the beginning not only of Croatian literacy but also of literature. The death of Dmitar Zvonimir also marked the end of the Croatian Trpimirovic dynasty, named after Prince Trpimir. Thus a struggle for the Croatian throne commenced. The Hungarian nobility gathered around Zvonimir's widow, Jelena, the sister of Hungarian King Ladislas, while the Croats gathered around Petar, a king chosen by the people. During these struggles the Zagreb diocese was established (1094), the Hungarian army invaded Croatia, and Petar was killed in this battle as the last Croatian king. However, the Hungarians could not take Croatia by force, so they signed a treaty in 1102, according to which the Croatian kingdom was attached to Hungary. In this arrangement the king was a common ruler to both Croatia and Hungary according to the principle of personal union. From that time a Croato-Hungarian state was created, in which Dalmatia, Croatia and Slavonia maintained their internal administration and elements of statehood. They had their bans (governors), viceroys, separate territory and even minted their own currency - the banovac.