In 917 the Serb prince Petar Gojniković, who was an ally of the Bulgarian Emperor openly changed sides and supported the Byzantines.
This danger from the rear played a significant role for the delay of the advance towards Constantinople after the striking victory at Anchialus.
In the fall of that year Simeon sent a punitive expedition force against the Serbs, led by Theodore Sigritsa and Marmais.
Marmais and Sigritsa lead a small army against Zaharije but they were ambushed and defeated, and their heads were later sent to the Byzantines.
The death of Marmais caused a major campaign against the Serbian principality which was defeated and annexed.