He commanded a corps of the "Grande Armée" in the Ulm campaign in which he distinguished himself. In the campaign of the middle Danube, which culminated in the Battle of Austerlitz, Napoleon placed him in command of the newly formed VIII Corps, composed of divisions from the other corps. Mortier over-extended his line of march on the north shore of the Danube and failed to heed Napoleon's advice to protect his north flank. A combined Russo-Austrian force, under the command of General Mikhail Kutuzov enticed Mortier to send General Théodore Maxime Gazan's 2nd Division into a trap and French troops were caught in a valley between two Russian columns. They were rescued by the timely arrival of a second division, under command of General Pierre Dupont de l'Étang's 1st Division, which covered a day's march in a half-day. The Battle of Dürrenstein (11 November 1805) extended well into the night. Both sides claimed victory, with the French losing more than a third of the participants, and Gazan's division experiencing over 40 percent losses. The Austrians and Russians also suffered heavy losses—close to 16 percent. After Austerlitz, Napoleon dispersed the corps and Gazan received the Legion of Honour, but Mortier was simply reassigned.