Croatia soon came into conflict with the Bulgarian Empire under Simeon I (called Simeon the Great in Bulgaria), who was already in a war with the Byzantines. Tomislav made a pact with the Byzantine Empire, for which he may have been rewarded by the Byzantine Emperor Romanos I Lekapenos with some form of control over the coastal cities of the Byzantine Theme of Dalmatia and with a share of the tribute collected from them. After Simeon conquered the Principality of Serbia in 924, Croatia received and protected the expelled Serbs with their leader Zaharija. In 926, Simeon tried to break the Croatian-Byzantine pact and afterwards conquer the weakly defended Byzantine Theme of Dalmatia, sending Duke Alogobotur with a formidable army against Tomislav, but Simeon's army was defeated in the Battle of the Bosnian Highlands. After Simeon's death in 927 peace was restored between Croatia and Bulgaria with the mediation of the legates of Pope John X. According to the contemporary "De Administrando Imperio", Croatian army and navy at the time could have consisted of approximately 100,000 infantry units, 60,000 cavaliers, and 80 larger ("sagina") and 100 smaller warships (called "condura"), but these numbers are generally taken as a considerable exaggeration. According to the palaeographic analysis of the original manuscript of "De Administrando Imperio", the population of medieval Croatia was estimated at between 440,000 and 880,000 people, while the military force was most probably composed of 20,000–100,000 infantrymen and 3,000–24,000 horsemen organized into 60 allagions.