Pope John X
During the war between the Byzantium and Bulgaria of Simeon I, in about 923, the Byzantines concluded an alliance with Croatia. Prior to that the Bulgarians had several decisive victories against the Byzantines, capturing Adrianople and endangering Constantinople. In 924 Simeon I deposed Zaharija from rule in Serbia, who fled to Croatia. In 926, Simeon's troops invaded Croatia, but were severely defeated in the Battle of the Bosnian Highlands. In 927 Pope John X sent his legates to mediate a peace treaty between Croats and Bulgarians.