Battle of Adrianople ( 1205 ) Background . The armies of the Fourth Crusade deviated from their stated goal of Jerusalem and instead captured and sacked the Christian city of Constantinople , the capital of the Byzantine Empire , in 1204 . The Bulgarian kingdom and Byzantine remnants soon united against the newly founded Latin Empire . The Battle . The Battle of Adrianople occurred on April 14 , 1205 between Bulgarians under Tsar Kaloyan of Bulgaria , and Crusade rs under Baldwin I . It was won by the Bulgarians after a skillful ambush using the help of their Cuman and Greek allies . Around 300 knights were killed , including Louis of Blois , Duke of Nicaea and Baldwin was captured and later died in captivity . The Bulgarians then overran much of Thrace and Macedonia . Baldwin was succeeded by his younger brother , Henry of Flanders , who took the throne on August 20 , 1206 . The main source document for this battle comes from the Chronicles of Geoffrey de Villehardouin . " Kaloyan , Tzar ( Emperor ) of Wallachia and Bulgaria , came to succour Adrianople with a very great army . He brought with him Bulgarians , Wallachians , and a full fourteen thousand Cumans who had never been baptised . " It was arranged by the Crusade rs that Geoffry the Marshal , and Manasses of l ' Isle should guard the camp , and that the Emperor Baldwin and all the remainder of the army should issue from the camp if Kaloyan came and offered battle . Thus they remained till the Wednesday of Easter week , and Kaloyan had by that time approached so near , that he encamped at about five leagues from the Crusaders . Kaloyan sent his Cumans running before the enemy camp . A cry was raised throughout the camp , and the crusaders rushed out to meet their foe . They pursued the lightly armored Cumans for a full league . The rout of the Cumans turned out to be a feigned rout , for when the Crusaders wanted to return to the camp , the Cumans turned around and began to shoot at them , wounding a good many of their horses . As the crusaders returned to the camp , the barons were summoned to the quarters of the Emperor Baldwin . They took counsel , and agreed that if Kaloyan would attack again , they would set themselves in array of battle before the camp , and not pursue the enemy when they turned around . The next day , the Thursday morning in Easter week , while the crusaders held mass , the Cumans again ran up to their tents . The Crusaders ran to arms , and issued from the camp with all their battalions in array , as they had agreed . Count Louis went out first with his battalion , and began to follow after the Cumans , and sent a messenger to urge the emperor to follow him . Count Louis followed in pursuit of the Cumans for at least two leagues , and caught up with them . The Cumans then turned upon them , and began shooting . Count Louis , who had been the first to attack , was wounded in two places and had fallen off his horse . With the Cumans and Wallachians closing in on the Crusader ' s ranks , one of his knights by the name of John of Friaise , dismounted , and set him back on his horse . Some of his knights pleaded Count Louis to return to the camp : " Sir , get you hence , for you are too sorely wounded , and in two places . " Louis responded : " The Lord God forbid that ever I should be reproached with fleeing from the field , and abandon the emperor . " The emperor , who was in great strains on his side , recalled his knights , telling them that he would not flee , and ordered his men to stay by his side . The desperate fight that followed lasted for several hours , but in the end , all Crusaders were slain or fled the field . On the battlefield remained the Emperor Baldwin , who swore to never flee , and Count Louis . The Emperor Baldwin was taken prisoner and Count Louis was slain . Among the slain crusaders were : Bishop Peter of Bethlehem , Stephen of Perche , brother to Count Geoffry , Renaud of Montmirail , brother of the Count of Nevers , Matthew of Wallincourt , Robert of Ronsoi , John of Friaise , Walter of Neuilli , Ferri of Yerres , John ' s brother , Eustace of Heumont and John ' s other brother , Baldwin of Neuville . In the aftermath , Bulgaria and Nicean Empire ( remains of Byzantine Empire after fall of Constantinople in 1204 - territory was situated in Asia Minor ) would form an alliance against the Latin Empire of Constantinople . The war against Kaloyan and Theodore Lascaris continued . In 1207 the Bulgarians attacked and killed Marquis Boniface of Montferrat at Messinopolis . He was beheaded and the head was sent to Kaloyan .