Bulgarian – Latin Wars The Bulgarian – Latin Wars were a series of conflicts between the Bulgarian Empire and the Latin Empire , which was created during the Fourth Crusade in 1204 . The wars were fought in the beginning of the 13th century . The initial expansionist ambitions of the Latin Empire were crushed only one year after its foundation after the Battle of Adrianople in 1205 , where its Emperor Baldwin I was captured and most of his knights perished . After that crucial defeat the Latin Empire had to defend itself against Bulgaria and the successor states of the Byzantine Empire , the Nicaean Empire in Asia Minor and the Despotate of Epirus in the Balkans . The rivalry between these three states postponed the end of the Latin Empire which occurred in 1261 . As a result of the conflicts the Bulgarian Empire expanded its territory taking control of most of the Balkan Peninsula while the influence of the Latin Empire was reduced to Constantinople and a few towns and islands . With the elimination of the Patriarchate of Constantinople by the Roman Catholic Crusaders , Bulgaria became the centre of the Orthodox Christianity . Foundation of the Latin Empire . On 13 April 1204 the knights of the Fourth Crusade seized the capital of the Empire of Romania ( Eastern Roman Empire ) , Constantinople , and replaced the ancient Byzantine Empire with a new Crusader state , the Latin Empire . Their leader Count Baldwin of Flanders was crowned Emperor in the Hagia Sophia as Baldwin I . According to the " Partitio Romaniae " he received a quarter of the Empire and the rest was divided between the Venetians and the Crusaders . The Emperor received the lands in Asia Minor as well as Constantinople and a thin strip along the Black Sea coast and a few other towns in the Balkans . The Venetians took the most fertile part of Byzantine Thrace including Adrianople ( Odrin ) , Rodosto , Arkadiopolis , most of the Peloponnese , parts of Epirus and Thessaly as well as many islands . Their Doge took the title " quartae partis et dimidiae totius imperii Romaniae dominator " or " Lord of a quarter and a half part of the whole Roman Empire " . The Crusaders received the other lands of the former Byzantine Empire and created the Kingdom of Thessalonica with Boniface of Montferrat selected for King . Bulgarian intervention . In 1204 , the Fourth Crusade captured Constantinople , capital of the Byzantine Empire , and created the Latin Empire , electing as emperor Baldwin I of Flanders . Although the Bulgarians had offered the crusaders an alliance against the Byzantine Empire , their offer had been spurned , and the Latin Empire expressed the intention of conquering all the lands of the former Byzantine Empire , including the territories ruled by Kaloyan , the Bulgarian emperor . The impending conflict was precipitated by the Byzantine aristocracy in Thrace , which rebelled against Latin rule in 1205 and called on Bulgarians for help , offering the Bulgarians its submission . Kaloyan ' s campaigns . As the Latin Emperor Baldwin I began to subdue rebel cities and besieged Adrianople , in the words of the Crusader chronicler Villehardouin , " Johannizza , King of Wallachia , was coming to succour Adrianople with a very great host ; for he brought with him Wallachians and Bulgarians , and full fourteen thousand Comans who had never been baptised " ( Villehardouin , 92 ) . On April 14 , 1205 , the Bulgarian Cumans managed to draw the pursuing heavy cavalry of the Latin Empire into an ambush in the marshes north of Adrianople , and the Bulgarians inflicted a crushing defeat on the crusader army . Emperor Baldwin I was captured , Count Louis I of Blois was killed , and the Venetian Doge Enrico Dandolo led the surviving portions of the crusader army into a hasty retreat back to Constantinople , during the course of which he died of exhaustion . ( Baldwin was imprisoned in the Bulgarian capital Tărnovo until he died or was executed later in 1205 . ) During the course of 1205 , Kaloyan captured Serres and Philippopolis ( Plovdiv ) , overrunning much of the territory of the Latin Empire in Thrace and Macedonia . However , the progress of the Bulgarians eventually was interrupted by the assassination of their emperor Kaloyan . On January 31 , 1206 the Bulgarians defeated the Latins again in Thrace , and later proceeded to capture Didymoteikhon . They repeatedly ravaged Thrace , including the important cities of Herakleia and Çorlu , and prompting the evacuation of other cities , such as Rodosto ( Tekirdağ ) . Whereas in the past the Bulgarian emperor , Kaloyan , had limited his oppression to the aristocracy , his later campaigns included wholesale transfer of populations from the captured cities to distant regions in Bulgaria . The Bulgarians besieged Adrianople twice , but failed to take the city because of the withdrawal of their Cuman cavalry , and the determined advance of the new Latin emperor , Baldwin I ' s brother Henry of Flanders . In 1207 the Bulgarians concluded an anti - Latin alliance with Theodore I Laskaris of the Empire of Nicaea . In the same year , the Bulgarians killed Boniface of Montferrat ( September 4 , 1207 ) , the Latin ruler of the Kingdom of Thessalonica . Seeking to take advantage of that situation , Kaloyan advanced on the city and besieged it with a large force , but was murdered by his own Cuman commander Manastăr at the beginning of October 1207 . In keeping with tradition , the Greek citizens of Thessalonica attributed the slaying of Kaloyan to their patron , Saint Demetrios . Peace . Kaloyan ’ s successor Boril , his nephew , was not nearly as successful against the Latins . Boril was defeated by Henry of Flanders in 1208 near Plovdiv but continued to campaign against the Latin Empire until 1210 , when the Hungarians and Latin empire made an alliance . Fall of the Latin Empire . By 1231 the Latin regency had finalized negotiations with John of Brienne , the former king of Jerusalem , who was invited to step in as the guardian and co - emperor of Baldwin II at Constantinople . This action led to the breach of the alliance between Bulgaria and the Latin Empire , and the creation of an alternate alliance between Bulgaria and Empire of Nicaea . However , the Bulgarian emperor Ivan Asen II could not decide whether to support the Niceaen Greeks or the Latins , and no decisive action was taken . Eventually Michael VIII Palaiologos , the ruler of the Niceaen Empire captured Constantinople and restored the Byzantine Empire , bringing an end to the Latin Empire .