Trelliard led the light cavalry of Marshal Jean Lannes' V Corps at the beginning of the War of the Third Coalition. He led the 9th and 10th Hussar Regiments at the Battle of Wertingen on 8 October 1805 during the Ulm Campaign. At the Battle of Austerlitz on 2 December, his command included the 9th and 10th Hussars and the 13th and 21st Chasseurs à Cheval. He fought on the left wing under Lannes.