After the stunning victory in the battle of Adrianople (1205) the Bulgarians gained control of most of Thrace except several larger cities which Emperor Kaloyan wanted to capture. In June 1205 he moved the theatre of the military actions to the south-west towards the domains of Boniface Montferrat, the King of Thessalonica and vassal of the Latin Empire.

The first town on the way of the Bulgarian army was Serres. The Crusaders tried to fight back in the vicinity of the town, but after the dead of the commander Hugues de Coligny were defeated and had to pull back to the town but during their retreat the Bulgarian troops also entered Serres. The remaining Latins under the command of Guillaume d'Arles were besieged in the citadel. In the negotiations which followed Kaloyan agreed to give them safe conduct to the Bulgarian-Hungarian border. However, when the garrison surrendered, the knights were killed while the ordinary people were spared.