In 917, a Byzantine army led by Leo Phokas invaded Bulgaria but was decisively defeated at the Battle of Achelous on 20 August 917. After the Achelous, Symeon sent an army to Serbia led by Pavle (after he had heard of a Byzantine–Serbian alliance), to take the Serbian throne, however, unsuccessfully as Petar proved a good opponent. Symeon then sent generals Marmais and Theodore Sigritsa, persuading Petar (through an oath) to come out and meet them, then captured and took him to Bulgaria where he was put in prison, dying within a year. Symeon put Pavle on the Serbian throne.