In 926 a battle was fought in the Bosnian highlands between the armies of the Bulgarian Empire, under the rule of Bulgarian Tsar Simeon I, who at the time also fought a war with the Byzantine Empire, and the Kingdom of Croatia under Tomislav, the first king of the Croatian state. The battle is also known as the Battle of the Bosnian Highlands (, ). It was fought in the Dinaric Alps of Eastern Bosnia near the rivers Bosna and Drina, the border area between the Kingdom of Croatia and the Bulgarian Empire. Principal information on the battle is provided by the emperor Constantine VII of the Byzantine Empire in his work De Administrando Imperio ("On the Governance of the Empire") and in the collection of preserved historical writings called Theophanes Continuatus. Simeon's aim was to defeat the Byzantine Empire and conquer Constantinople. To achieve his aim, Simeon overran the eastern and central Balkans several times, occupied Serbia and finally attacked Croatia. The result of the battle was an overwhelming Croatian victory. Background Events preceding the war After the war between Trpimir I and Bulgarian Knyaz Boris I in 853, which resulted in a peace treaty, the relations between Bulgaria and Croatia improved greatly. Ambassadors from Rome regularly went through Croatian territory to Bulgaria and received escorts to the border, while the Pope had regular conversations with both countries. Croatia bordered Bulgaria probably somewhere in present-day Bosnia, between the rivers Bosna and Drina. The situation started changing in the early 10th century when the new Bulgarian ruler Simeon I started a campaign against the Byzantine Empire. The conflict went in favor of Bulgaria and the Byzantines found themselves in great danger. Simeon captured a large part of the Byzantine territory in Europe and was crowned as "Emperor of the Bulgarians" by Patriarch Nicholas Mystikos in 913. He was later crowned at the church of Ohrid as "Tsar of all Bulgarians and Greeks" by the newly appointed Bulgarian patriarch in 925. However, the Byzantines always addressed Simeon as "prince" (archon) and the prelate continued to be referred to as an archbishop. According to the juridical reasoning of the time, only the Pope and the Byzantine Emperor could bestow royal or imperial titles, and an emperor might be crowned only by a patriarch. Byzantine Emperor Romanos I Lekapenos protested bitterly against Simeon's usurpation of the imperial title. The Patriarch of Constantinople, Nicholas