The Battle of Adrianople (in 1205) took place on April 13 and April 14, 1205 AD. In it the forces of the Second Bulgarian Empire (1185-1396 AD) under Tsar Kaloyan (r. 1197-1207) routed the knights of the Fourth Crusade from the Latin Empire (1204-1261 AD) under Emperor Baldwin I on Constantinople, also known as Baldwin of Flanders.
The Battle of Adrianople in 1205 resulted from the growing enmity between the Western European knights from the Fourth Crusade and the Bulgarian Empire, after in 1204 the Crusaders deviated from their goal of taking Jerusalem, and instead ransacked Constantinople, the capital of the Byzantine Empire, founding their Latin Empire. Even though by that time the Bulgarian Tsar Kaloyan had established good relations with Pope Innocent III in Rome who organized the Fourth Crusade, the Western European (mostly French) knights demanded the obedience of the Bulgarian Tsar arguing that since they had taken Constantinople, they had a right to the entire supposedly Byzantine domain, including all of Bulgaria, which had liberated itself from Byzantium 20 years earlier.
The conquest of Constantinople itself occurred after in January 1203, en route to Jerusalem, the Crusaders made an agreement with Byzantine prince Alexios Angelos to restore his deposed father as emperor. In August 1203, after clashes outside Constantinople, Alexios Angelos was crowned as co-Emperor with Crusader support as Alexios IV alongside his blinded father, Isaac II Angelos (r. 1185-1195, 1203-1204 AD). However in January 1204, Alexios IV was deposed by a popular uprising in Constantinople, and was murdered on 8 February 1204. In April 1204, the discontent Crusaders captured and brutally ransacked the city setting up the Latin Empire, and partitioning the conquered Byzantine territories among themselves as feuds.
The 1205 Battle of Adrianople was won by the Bulgarians and their Cuman allies, after an ambush. The Bulgarian forces are estimated to have been around 40,000 troops, including 14,000 lightly armed Cuman cavalry. The Crusaders’ forces are also estimated at tens of thousands, including 300 West European heavy mounted knights. Most of the 300 Western European knights were killed in the Battle of Adrianople, making it one of the greatest defeats ever suffered by Crusaders.
Latin Emperor Baldwin I of Constantinople (Baldwin of Flanders) was taken prisoner and died in captivity after a year later. He was kept in a fortress tower in the then Bulgarian capital Tarnovgrad (today’s Veliko Tarnovo) which is known to this day as Baldwin’s Tower. It is unknown how exactly he died, especially since it is believed that he was treated well at first. There are stipulations that he was killed by the Bulgarian Tsar Kaloyan in a fit of rage, possibly over a revolt in Philipopolis (today’s Plovdiv) which led the city to be surrendered to the Latin Empire. A Bulgarian legend has it that Baldwin found his death after trying to seduce the Bulgarian Tsaritsa (i.e. empress).
After the Battle of Adrianople, the Second Bulgarian Empire overran most of the geographic regions of Thrace and Macedonia. The first Latin Emperor Baldwin was succeeded by his younger brother, Henry of Flanders, who took the throne on August 20, 1206. Bulgaria and the Nicean Empire (one of the three successor states of Byzantium, which ultimately restored it in 1261 AD) made an alliance against the Latin Empire. In 1207, the Bulgarians attacked and killed Marquess Boniface of Montferrat, a feudal lord with French and Italian estates who in 1202 AD became the leader of the Fourth Crusade and King of Thessaloniki (r. 1205-1207). He was beheaded at Messinopolis, and his head was sent to Tsar Kaloyan. One of the major written sources for the Battle of Adrianople is the Chronicles of Geoffrey de Villehardouin, a French knight and historian who chronicled the Fourth Crusade.