In 1809, he returned to Europe via Russia, and served for a while as a volunteer in the Polish army of the Duchy of Warsaw. Arriving at Vienna, he was named captain in the French Imperial Guard. He served as aide-de-camp of Marshal Auguste de Marmont in Spain, and was sent by him to Russia to inform Napoleon on the Battle of Salamanca. He arrived at Napoleon's headquarters on 6 September 1812, the eve of the Battle of Borodino. Fabvier was gravely wounded in this battle, leading the charge during the final assault on the Russian fortifications. Napoleon rewarded him by naming him artillery major in the VI Corps under Marshal Ney. He distinguished himself further during the operations of the War of the Sixth Coalition in Germany, and was raised to colonel of the General Staff and made "baron de l'Empire". He participated in the retreat into France, and on 31 March 1814, on behalf of Marshals Marmont and Mortier, he signed the surrender of Paris to the Coalition armies.