To overcome his failure to secure gains against the Serbians, Andronikos III attempted to annex Bulgarian Thrace, but the new tsar Ivan Alexander of Bulgaria defeated Byzantine forces at Battle of Rusokastro on 18 July 1332. In the summer of the same year, the Byzantines gathered an army and without a declaration of war advanced towards Bulgaria, looting and plundering the villages on their way. The Byzantines seized several castles because Ivan Alexander's attention was focused towards fighting the rebellion of his uncle Belaur in Vidin. He tried to negotiate with the enemy without success. The Emperor decided to act swiftly during the course of five days, when his cavalry covered 230 km to reach Aitos and face the invaders.

The battle began at six in the morning and continued for three hours. The Byzantines tried to prevent the Bulgarian cavalry from surrounding them, but their manoeuvre failed. The cavalry moved around the first Byzantine line, leaving it for the infantry and charged the rear of their flanks. After a fierce fight the Byzantines were defeated, abandoned the battlefield and took refuge in Rusokastro.