Baldwin I, the first emperor of the Latin Empire of Constantinople, as Baldwin IX Count of Flanders and as Baldwin VI Count of Hainaut, was one of the most prominent leaders of the Fourth Crusade, which resulted in the sack of Constantinople and the conquest of large parts of the Byzantine Empire, and the foundation of the Latin Empire, also known as Romània. He lost his final battle to Kaloyan, the emperor of Bulgaria, and spent his last days as his prisoner.
Baldwin was the son of Baldwin V of Hainaut and Margaret I, Countess of Flanders. In 1194, Baldwin took possession of a much-reduced Flanders, lessened by the large chunk, including Artois, given by Philip of Alsace as dowry to Baldwin's sister Isabelle of Hainaut, and another significant piece to his own wife. Isabelle had died in 1190, but King Philip still retained her dowry, on behalf of Isabelle's son, the future Louis VIII of France. The eight years of Baldwin's rule in Flanders were dominated by his attempts to recover some of this land.
On Ash Wednesday (23 February) 1200 in the town of Bruges, Baldwin took the cross, meaning he committed to embark on a crusade. He spent the next two years preparing, finally leaving on 14 April 1202.
The leaders of the Fourth Crusade were persuaded to diverted to Constantinople in large part due to the exiled Byzantine prince Alexios (future Emperor Alexios IV Angelos) who promised them supplies and money in return for their help in ousting his uncle Emperor Alexios III Angelos, and freeing his father Isaac II Angelus. In April 1204, after numerous negotiations attempting to obtain the promised funds from the Byzantines, the Crusaders conquered the most powerfully protected city in the world. Stunned at their own success and unsure of what to do next, the leaders adopted a similar track as their forefathers had during the First Crusade. They elected one of their own, Count Baldwin of Flanders as Emperor (of what modern historians refer to as the Latin Empire) and divided imperial lands into feudal counties.
During the following winter (1204–1205) the Franks prosecuted conquests in Bithynia, in which Henry, Baldwin's brother, took part. But in February the Greeks revolted in Thrace, relying on the assistance of Kaloyan, tsar of Bulgaria, whose overtures of alliance had been rejected by the emperor. The garrison of Adrianople was expelled. Baldwin along with Dandolo, the count of Blois, and Marshal Villehardouin, the historian, marched to besiege that city. The Frankish knights were defeated (14 April 1205); the count of Blois was slain, and the emperor captured by the Bulgarians.
For some time his fate was uncertain, and in the meanwhile Henry, his brother, assumed the regency. Not until the middle of July the following year was it ascertained that he was dead. The circumstances of Baldwin's death are not exactly known. It seems that he was at first treated well as a valuable hostage, but was sacrificed by the Bulgarian monarch in a sudden outburst of rage, perhaps in consequence of the revolt of Philippopolis, which passed into the hands of the Franks. According to a Bulgarian legend, Baldwin had caused his own downfall by trying to seduce Kaloyan's wife. The historian George Acropolites reports that the Tsar had Baldwin's skull made into a drinking cup, just as had happened to Nicephorus I almost four hundred years before.
Tsar Kaloyan wrote to Pope Innocent III, reporting that Baldwin had died in prison. A tower of the Tsarevets fortress of the medieval Bulgarian capital, Veliko Tarnovo, is still called Baldwin's Tower; supposedly, it was the tower where he was interned. It was not until July 1206 that the Latins in Constantinople had reliable information that Baldwin was dead. His brother Henry was crowned emperor in August.