Theoderic (Dutch: Diederik, French: Thierry, German: Dietrich; c. 1099 – 17 January 1168), commonly known as Thierry of Alsace, was the fifteenth count of Flanders from 1128 to 1168. With a record of four campaigns in the Levant and Africa (including participation in the Second Crusade, the failed 1157–1158 siege of the Syrian city Shaizar, and the 1164 invasion of Egypt), he had a rare and distinguished record of commitment to crusading.
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Countship
Theoderic was the youngest son of Duke Theoderic II of Lorraine and Gertrude, daughter of Count Robert I of Flanders. After the murder of his cousin, Charles the Good, in 1127, Theoderic claimed the County of Flanders, but another cousin, William Clito, became count instead with the support of King Louis VI of France. William's politics and attitude towards the autonomy of Flanders made him unpopular, and by the end of the year Bruges, Ghent, Lille, and Saint-Omer recognized Theoderic as a rival count. Theoderic's supporters came from the Imperial faction of Flanders.
Louis VI of France had Raymond of Martigné, the Archbishop of Reims, excommunicate Theoderic. Louis VI then besieged Lille, but was forced to retire when Henry I of England, William Clito's uncle, transferred his support to Theoderic. However, Theoderic was defeated at Axspoele and fled to Bruges. He was forced to flee Bruges as well, and went to Aalst, where he was soon under siege from William, Godfrey I of Leuven, and Louis VI. 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.