Commanding nine squadrons of the 9th and 10th Hussars and the 21st Chasseurs at the outbreak of the War of the Fourth Coalition, Trelliard fought at the Battle of Saalfeld on 10 October 1806. During this action, Quartermaster Guindet of the 10th Hussars killed Prince Louis Ferdinand of Prussia in personal combat. Trelliard led his cavalrymen at the Battle of Jena on 14 October. Lannes posted his cavalry on the right wing at the Battle of Pultusk on 26 December. Lured into attacking Russian hussars, Trelliard's cavalry suffered serious loss when the enemy cavalry wheeled aside, exposing the French to the fire of a masked battery. He was badly wounded in the action. In the same month he received promotion to general of division.