Petar of Serbia, although allied to Simeon, has become increasingly disgruntled by the fact that he is essentially subordinate to him.

Peter’s expansion toward the coast facilitates contacts with the Empire, by way of the strategos of Dyrrhachium.

Searching for allies against Bulgaria, Constantinople has showered Peter with gold and promises of greater independence if he will join their alliance.

Petar might have been planning an attack on Bulgaria with the Magyars, showing that his realm had stretched north to the Sava river.

However, Michael of Zachumla has forewarned Simeon of this plan, since Michael is an enemy of Peter, and a loyal vassal of Simeon.

Simeon attacks Serbia in 917 and deposes Peter, placing Pavle Branović (a grandson of Mutimir) as Prince of Serbia, subordinate to Simeon (although some scholars suggest that Symeon took control over Serbia directly at this time.)

Constantinople, unhappy with Simeon’s intervention in Serbia, sends Zaharije Prvoslavljevic in 920 to oust his cousin Pavle, but he fails and is sent to Bulgaria as prisoner.

Simeon arrives at Constantinople in the summer of 924 and demands to see the patriarch and the emperor.

He converses with Romanos on the Golden Horn on September 9, 924 and arranges a truce, according to which Constantinople will pay Bulgaria an annual tax, but will be ceded back some cities on the Black Sea coast.

During the interview of the two monarchs, two eagles are said to have met in the skies above and then to have parted, one of them flying over Constantinople and the other heading to Thrace, as a sign of the irreconcilability of the two rulers.

In his description of this meeting, Theophanes Continuatus mentions that "the two emperors... conversed", which may indicate renewed Constantinople’s recognition of Simeon's imperial claims.

Simeon's troops under Alogobotur invade Croatia in 926, at this time an ally of Constantinople.

The reason might have been that Tomislav had received and protected the Serbs who were expelled by Simeon from Rascia.

In all probability, however, the main reason is that Simeon, if crowned by the Papal Legate, fears an attack from the Emperor Romanos Lekapenos, supported by Tomislav.

Romanos had won the friendship of Tomislav some years previously, handing over Dalmatia to Tomislav and recognizing him as King of Croatia (Pope John X had recognized Tomislav as King of Croatia in 925).

Tomislav had sent his troops to Italy during the summer of 926 to expel Saracens from the city of Sipontus, which belongs to the imperial province of Langobardia.

This event could have been sufficient proof to Simeon that the Croats had taken the side of Constantinople and that they would support the Greek emperor actively in the future.

Therefore, when Simeon sends a great army against the Croats, the Bulgarians are met by Tomislav's army in the mountainous region of Eastern Bosnia.

The Croatian forces under the leadership of their king completely devastate the Bulgarian army.

Key to Tomislav's triumph is likely the choice of terrain on which the battle takes place: Croatian soldiers are probably more skilled in fighting in the mountainous terrain of the Bosnian highlands.

The Croatian victory is so decisive and the battle so big that contemporary sources greatly overestimate Croatia's army at one hundred and sixty thousand men, with a slightly bigger force on the Bulgarian side.

This is the only battle Tsar Simeon ever loses.

Since both rulers maintain good relations with John X, the pope is able to negotiate an end to the war soon afterward without any further border changes.

Tomislav, fearing Bulgarian retribution, accepts to abandon his union with Constantinople and make peace on the basis of the status quo, negotiated by the papal legate Madalbert.