King Louis Bonaparte put him in command of the Dutch brigade (part of the Division-Leval) that his brother Napoleon obliged him to contribute to the French campaign in Spain in 1808. For his exploits in that harsh guerilla war in the battles of Battle of Zornoza, Mesas de Ibor (1809), Talavera (1809), Almonacid (1809) and Ocana (1809; where he assumed command of the Division-Leval), King Louis created Chassé baron on 1 July 1810, just one week before Napoleon annexed the Kingdom of Holland to the French Empire. Like many Dutchmen, Chassé resented this annexation, so much so, that he refused to accept the "diplome" in which Napoleon elevated him to "baron de l'Empire" on 30 June 1811. Nevertheless, he continued serving in the army that now became part of the imperial French army.