[3] In June or July 1197, Thierry witnessed the treaty of alliance between Baldwin and King Richard I of England.
Following the lead of his cousin, he joined the Fourth Crusade in 1202 and was one of the commanders of the Flemish fleet alongside John II of Nesle and Nicholas of Mailly.
[4] The fleet sailed in the summer of 1202, attacked an unnamed Muslim city on the African coast and overwintered in Marseille.
[6] At Marseille, the fleet received news of the planned attack on Constantinople and Baldwin ordered it to rendezvous with him off Methoni.
[8] In July or August 1207, Thierry, his cousin Eustace and Anseau de Cayeux were sent by the Emperor Henry to raid Bulgaria.