Ivan Asen II of Bulgaria
He began his reign during a very critical period in the history of the Latin Empire of Constantinople, because the fall of the Kingdom of Thessalonica in 1224 had interposed formidable enemies between the capital of the empire on the one hand and the crusaders' states in the Peloponnese on the other. But the defeat of Emperor Theodore Komnenos Doukas of Thessalonica ("c." 1225-1230) by Tzar Ivan Asen II of Bulgaria (1218–1241) at Klokotnitza in April, 1230 freed them from the dangers inherent in the great concentration of power in the hand of Theodore Komnenos Doukas.