Around the same time, Tsar Kaloyan, the Tsar of Bulgaria, successfully completed negotiations with Pope Innocent III. The Bulgarian ruler was recognized as "rex", i.e. emperor (tsar), while the Bulgarian archbishop regained the title "primas", a title equal to that of patriarch.

Despite the apparently good relations between Tsar Kaloyan and the new Western European conquerors, immediately after settling down in Constantinopole, the Latins stated their pretensions on Bulgarian lands. Latin knights began crossing the border to pillage Bulgarian towns and villages. These belligerent actions convinced the Bulgarian Emperor that an alliance with the Latins was impossible and that it was necessary to find allies from among the Greeks of Thrace that had yet to be conquered by the knights. In the winter of 1204-1205 messengers of the local Greek aristocracy visited Kaloyan and an alliance was formed.

The Battle of Adrianople occurred around Adrianople on April 14, 1205 between Bulgarians, Vlachs and Cumans under Tsar Kaloyan of Bulgaria, and Crusaders under Baldwin I, who only months before had been crowned Emperor of Constantinople, allied with Venetians under Doge Enrico Dandolo. The battle was won by the Bulgarian Empire after a successful ambush. The main part of the Latin army is eliminated, the knights are defeated and their emperor, Baldwin I, is taken prisoner in Veliko Tarnovo.