Battle of Drava River ( 10th century ) According to the " Chronicle of the Priest of Dioclea " from the late 12th century , Tomislav of Croatia defeated the Hungarians in battle . Others question the reliability of this account , because there is no proof for this interpretation in other records . Slavonia should have been an integral part of the medieval Croatian state , if the battle had happened , however according to the Byzantine ruler Constantine Porphyrogenitus Slavonia was clearly under Hungarian control . The exact place and time of the battle is not known . Background , battle and consequences . Having arrived in the Pannonian Plain by the end of the 9th century and conquering the people living north of the Drava River , Hungarians intensified their fast looting raids across continental Europe . They started occasionally to perform devastating raids and military campaigns to the south as well , across the Drava river , to the territory of the Principality of Pannonian Croatia . They succeeded in defeating the Principality ' s last known ruler Braslav ( ruled in 880 – 898 / 900 ) , who was vassalaged to the Kingdom of East Francia . Somewhere at the beginning of the 10th century , Hungarians went further south and attacked the Principality of Dalmatian Croatia , then ruled by Muncimir ( 892 – 910 ) and his successor Tomislav ( 910 – 928 ) , two Knezes ( Princes or Dukes ) from the House of Trpimirović . Tomislav undertook measures to prevent Hungarian incursions , mobilizing his army , deploying to the north and carrying out the military actions to fight the enemy . According to meager medieval sources , the decisive battle took place in an undefined area of the right , southern bank of the Drava River . Historians have come to such conclusion on the basis of the following sources : Chronicle of the Priest of Dioclea from the 12th century ( Chapter XII : " „ . . . King Tomislav , brave young man and powerful warrior , . . . fought many wars with the Hungarian king and always forced him to flee . . . “ " ) , De Administrando Imperio from the 10th century , Gesta Hungarorum from the 12th century and Historia Salonitana from the 13th century . There are no contemporary accounts of the battle . In „ De Administrando Imperio “ , written a few decades after the battle by Byzantine Emperor Constantine VII Porphyrogenitus , there are more data about another battle Tomislav of Croatia fought , the Battle of the Bosnian Highlands , against the army of the mighty Bulgarian Empire in 927 , which resulted in Croatian decisive victory . Together with Constantine ' s assertion that Tomislav was able to field an army of 100 , 000 foot soldiers and 60 , 000 horse soldiers ( which numbers are disputed though ) , it shows however the strength and capability of the Croatian army , able to reject Hungarians . After the battle , Tomislav took control over the territory south of Drava , uniting Croatian lands from the Adriatic Sea in the south to the Drava River in the north , as well as from eastern part of Istria ( Raša River ) in the west to Drina River in the east . Moreover , the Croatian ruler governed the coastal towns of Byzantine Province of Dalmatia , which he was given by the Emperor . The records of the two Synods of Bishops held in Split in 925 and 928 indirectly confirm that Croatia comprised the territory of medieval Slavonia , with its capital Sisak . At the 925 Synod Gregory of Nin lost his diocese and was offered to choose another one , among the temporary vacant bishop seats of Skradin , Sisak or Delmit ( possibly Delminium - present - day Tomislavgrad , maybe Omiš ) . Just the possibility to choose Sisak , the only bishopric in Slavonia at that time and the seat of the ruler of Pannonian Croatia , confirms that the area of this former principality was after the battle of Drava controlled by King Tomislav of Croatia .