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.

The Battle of Klokotnitsa occurred on 9 March 1230 near the village of Klokotnitsa between Second Bulgarian Empire and Empire of Thessalonica. As a result, Bulgaria emerged once again as the most powerful state in South-Eastern Europe. Nevertheless, Bulgarian power was soon to be contested and surpassed by the rising Empire of Nicaea. The Epirote threat to the Latin Empire was removed. Thessalonica itself became a Bulgarian vassal under Theodore's brother Manuel.