In the reign of Kaloyan, the Greek noblemen of eastern Thrace had risen up against the Bulgarian Empire, seeking assistance from the Latin Empire; this rebellion would continue against the new Emperor of Bulgaria Boril, who continued the war of his predecessor Kaloyan against the Latin Empire invaded Eastern Thrace. During his march, he seized parts of Alexius Slav's territory before stopping at Stara Zagora.

The Latin Emperor Henry gathered an army in Selymbria and headed to Adrianople. The battle of Beroia took place in June 1208 near the city of Stara Zagora, Bulgaria between the Bulgarians and the Latin Empire. It resulted in a Bulgarian victory. he retreat continued for twelve days, in which the Bulgarians closely followed and harassed their opponents inflicting casualties mainly to the Latin rear-guard which was saved several times from complete collapse by the main Crusader forces. However, near Plovdiv the Crusaders finally accepted the battle and the Bulgarians were defeated.