Around 1221–1222 Emperor Ivan Asen II of Bulgaria made an alliance with Theodore Komnenos Doukas, the ruler of Epirus. Secured by the treaty, Theodore managed to conquer Thessalonica from the Latin Empire, as well as lands in Macedonia including Ohrid, and establish the Empire of Thessalonica. After the death of the Latin emperor Robert of Courtenay in 1228, Ivan Asen II was considered the most probable choice for regent of Baldwin II. Theodore thought that Bulgaria was the only obstacle left on his way to Constantinople and in the beginning of March 1230 he invaded the country, breaking the peace treaty and without a declaration of war.

Theodore Komnenos summoned a large army, including western mercenaries. He was so confident of victory that he took the whole royal court with him, including his wife and children. His army moved slowly and plundered the villages on its way. When the Bulgarian tsar learned that the state was invaded, he gathered a small army of a few thousand men including Cumans and quickly marched southwards. In four days the Bulgarians covered a distance three times longer than Theodore's army had travelled in a week.

On 9 March, the two armies met near the village of Klokotnitsa. It is said that Ivan Asen II ordered the broken mutual protection treaty to be stuck on his spear and used as a flag. He was a good tactician and managed to surround the enemy, who were surprised to meet the Bulgarians so soon. The battle continued until sunset. Theodore's men were completely defeated, only a small force under his brother Manuel managed to escape from the battlefield. The rest were killed in the battle or captured, including the royal court of Thessalonica and Theodore himself. 

Ivan Asen II immediately released the captured soldiers without any conditions and the nobles were taken to Tarnovo. His fame for being a merciful and just ruler went ahead of his march to the lands of Theodore Komnenos and Theodore's recently conquered territories in Thrace and Macedonia were regained by Bulgaria without resistance.