Merlin led cavalry at the Battle of Almonacid on 11 August 1809. The IV Corps cavalry on that occasion included the 3rd Dutch Hussars and the Polish Lancers of the Legion of the Vistula. King Joseph appointed Merlin to command his Royal Guard on 16 August, but he also led the light cavalry in the rout of the Spanish at the Battle of Ocaña on 18–19 November 1809 according to historian Charles Mullié. This is contradicted by Charles Oman who asserted that Antoine-Marie Paris d'Illins led the IV Corps cavalry at Ocaña. Paris was killed in a cavalry melee on 18 November and was succeeded by André Thomas Perreymond.