[1] When the childless Count Philip I of Flanders left on the last of his personal crusades in 1177, he designated Baldwin, his brother-in-law, as his heir.

Philip died without further issue of disease on the Third Crusade at the siege of Acre in 1191, he was succeeded in Flanders by Baldwin V of Hainaut, although the two had been on seemingly uncordial terms since the 1186 treaty.

[6] Through Marie, Baldwin had additional connections and obligations to the defenders of the Holy Land: her brother Henry II of Champagne had been King of Jerusalem in the 1190s (leaving a widow and two daughters who needed help to keep and regain their territories in Palestine).

A month after the treaty, on Ash Wednesday (23 February) 1200 in the town of Bruges, Baldwin took the cross, meaning he committed to embark on a crusade.

As part of his effort to leave his domains in good order, Baldwin issued two notable charters for Hainaut.

Marie was regent for Baldwin for the two years she remained in Flanders and Hainaut, but by early 1204, she had left both her children behind to join him in the East.

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.

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.

While Boniface was considered the most probable choice, due to his connections with the Byzantine court, Baldwin was young, gallant, pious, and virtuous, one of the few who interpreted and observed his crusading vows strictly, and the most popular leader in the host.

[14] During his coronation, Baldwin wore a very rich jewel that had been bought by Byzantine Emperor Manuel I Komnenos for 62,000 silver marks.

[15] His own special portion consisted of the city of Constantinople, the adjacent regions both on the European and the Asiatic side, along with some outlying districts, and several islands including Lemnos, Lesbos, Chios and Tenos.

In this enterprise in the summer of 1204, Baldwin came into collision with Boniface of Montferrat, the rival candidate for the empire, who received a large territory in Macedonia with the title of king of Thessalonica.

Boniface hoped to make himself quite independent of the empire and do no homage to his kingdom, and he opposed Baldwin's proposal to march to Thessalonica.

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 Frankish knights were defeated (14 April 1205); the count of Blois was slain, and the emperor was captured by the Bulgarians (see Battle of Adrianople).

It seems that he was at first treated well as a valuable hostage, but was executed 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.

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.