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 of 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. 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==