[2] William's politics and attitude towards the autonomy of Flanders made him unpopular,[2] and by the end of the year Bruges, Ghent, Lille, and Saint-Omer recognized Theoderic as a rival count.

[3] Louis VI then besieged Lille, but was forced to retire when Henry I of England, William Clito's uncle, transferred his support to Theoderic.

The city was about to be captured when William died on 28 July 1128 from an infected wound sustained during the siege, leaving Theoderic as the only claimant to the seat.

Theoderic himself swore homage to Louis VI after 1132,[4] in order to gain the French king's support against Baldwin IV, Count of Hainaut, who had advanced his own claim on Flanders.

He soon returned to Flanders to put down a revolt in the Duchy of Lower Lotharingia, ruled at the time by Godfrey III of Leuven.

He led the crossing of the Maeander River in Anatolia and fought at the Battla of Attalya in 1148, and after arriving in the crusader Kingdom he participated in the Council of Acre, where the ill-fated decision to attack Damascus was made.

He participated in Baldwin III's siege of Shaizar in 1157,[7] but the fortress remained in Muslim hands when a dispute arose between Theoderic and Raynald of Châtillon over who would possess it should it be captured.

There had also been great economic and agricultural development, and new commercial enterprises were established; Flanders' greatest territorial expansion occurred under Theoderic.