Battle of the Bosnian Highlands

In 926, after mounting conflict between Tsar Simeon the Great and the Croatian king Tomislav, war broke out between the two states. 

The decisive engagement was the Battle of the Bosnian Highlands, fought on 27 May 927. Simeon sent a large army against the Croats, which was met by Tomislav's army. The battle took place in the mountains of eastern Bosnia, near the Drina and Bosna rivers. These areas formed the border zone between Bulgaria and the Kingdom of Croatia. According to Constantine Porphyrogenitus, King Tomislav commanded a vast army of 160,000 men. Meanwhile, no information survives about the number of soldiers in Simeon's army, but it is estimated at 30,000–70,000.

The Croatian forces under Tomislav completely defeated the Bulgarian forces, killing most of the Bulgarians in the battle. The Croatian victory was so decisive and the battle so large that contemporary sources greatly overestimated the size of the Croatian army at 160,000 men, as well as the Bulgarian army, which was slightly larger than the Croatian force. This was the only battle Simeon ever lost. Both rulers maintained good relations with Pope John X, who attempted to negotiate peace without territorial changes.

On the day of the battle, Simeon died in Preslav, and his successor Peter I faced internal difficulties and rebellions by his brothers Michael and Ivan. The Serbs took advantage of this situation and in 931 returned to their homes in the re-established Serbian principality, thereby destroying Bulgaria's short-lived neighbour.