Croatian – Bulgarian Wars The Croatian – Bulgarian Wars were a series of conflicts that erupted three times during the 9th and 10th centuries between the medieval realms of Croatia and Bulgaria . During these wars , Croatia formed alliances with Eastern Francia and Byzantium against the Bulgarian Empire . First war . During the middle of the 9th century , Bulgaria was the dominant power in the central , eastern , and northern Balkans . In 854 , the Bulgarian ruler Boris I forged an official alliance with the Moravian prince Rastislav against Louis the German of East Francia . Duke Trpimir of Croatia was a faithful Frankish vassal and was wary of Bulgaria ' s ongoing expansion when it reached Croatian borders after the wars against Rascia . Bulgaria is said to have invaded Croatia in approximately 854 , but there is also a possibility that King Louis gave some compensation to Trpimir to attack Bulgaria . During the war of 854 , there was only one great battle on the territory of present - day northeastern Bosnia , and neither side emerged victorious . Soon afterward , peace negotiations began between Boris of Bulgaria and Trpimir of Croatia , resulting in gifts exchanged and the establishment of peace , with the border between the Croatian - Dalmatian Duchy and Bulgaria stabilized at the river of Drina ( between modern - day Bosnia and Herzegovina and Republic of Serbia ) . Second war . A very long and arduous war was prosecuted between the Bulgarian Tsar Simeon I , who wanted to take over the Byzantine Empire , and the Empire ' s monarch Romanos I . Under heavy pressure from the Bulgarians and amassing defeat after defeat , the Byzantine Empire negotiated with Serbia and Croatia in an effort to forge an alliance against the Bulgarians . Having been informed of these plans by Prince Michael of Zachlumia , who was forced to the islands as the Serbian rulers took direct control most of his local lands for themselves , Simeon overran Serbia in 924 , destroyed it by tricking its nobility , and made it directly part of the Bulgarian Empire . Serbia ' s ruler Zaharija Pribislavljević fled and found exile at the Croatian court , while after the realm ' s destruction massive waves of Serb refugees fled and found refuge in Tomislav ' s Croatia . Trying to gain Croatian entry into this coalition , Romanos I ordered the province of Dalmatia in ca . 925 to pay taxes not to Byzantium but rather to the Croatian state ; the Dalmatian territories , including most cities and the northern islands , were henceforth administered by King Tomislav and were to remain under Croatia . The Serbs of Zahumlje , allied with Croatia and Prince Michael , became vassals of Tomislav some time before 926 . Tsar Simeon sent Duke Alogobotur to drive the Serb refugees further into Croatia , causing outright war in ca . 926 . The climax of the war was the Battle of the Bosnian Highlands that same year , when Croatian forces under the command of King Tomislav completely defeated the Bulgarian forces under the command of Alogobotur , killing most of the Bulgarians in the battle . The Croatian victory was so decisive and the battle so big that contemporary sources greatly overestimated Croatia ' s Army at 160 , 000 men , with a slightly bigger force on the Bulgarian side . This was the only battle Tsar Simeon ever lost . Since both rulers maintained good relations with Pope John X , the pope was able to negotiate an end to the war soon afterward without any further border changes . Although the eastern border was extended down to the river of Bosnia , the Croatian realm was greatly strengthened both militarily and in natural resources : it emerged from the war as one of the most modern realms of the period and was able to consolidate a moderately sized navy . On the same day as the battle Simeon died in Preslav and his successor Peter I faced internal difficulties and revolts by his brothers Michail and Ivan . The Serbs were able to take advantage of this situation and many of them returned by 931 to their homes in the renewed Serbian realm , thus terminating Croatia ' s short - lived Bulgarian neighbor at the east . Third war . In the second half of the 10th century , the Croatian realm was ruled by Stjepan Držislav . Stjepan formed an alliance with the Byzantine Empire , which in turn recognized him as the King of his lands . After his death in 997 , his son Svetoslav Suronja continued his pro - Byzantine policy . His brothers Krešimir III and Gojslav did not want Svetoslav as ruler and attempted a coup d ' etat , asking the Bulgarian tsar Samuil for help . Answering their call , Samuil went on a rampage in 998 and razed the Dalmatian cities of Trogir and Split , but he was stopped at the siege of Zadar . The Bulgarian forces then returned home , using the Bosnian route . Territory taken by Samuil during the war was given to Krešimir III and Gojslav , who , with further Bulgarian support , won the Croatian civil war and took over the realm in the year 1000 . Svetoslav Suronja , a Byzantine and Venetian ally , was sent to exile in Venice , but after a change in government there he was exiled to Hungary . Following the death of Ivan Vladislav in 1018 , Bulgaria fell under Byzantine rule , and Krešimir III and Gojslav , the two Croatian kings , became Byzantine vassals .