Colbert distinguished himself at the Battle of Elchingen on 14 October 1805. During the combat, he led the 3rd Hussar and 10th Chasseur Regiments in support of Marshal Michel Ney's attacking infantry. He also fought at the Battle of Austerlitz in December. Promoted to Brigadier General at the end of 1805, he was given an important mission to St Petersburg by Napoleon, where he was accompanied by his great friend Claude Testot-Ferry, later a colonel in the cavalry of the Imperial Guard. He and Ferry met again in Spain. He married the daughter of senator Canclaux, and they had 2 sons in 1805 and 1808.