Merlin was promoted
general of brigade on 1 February 1805. In Masséna's Army of Italy, he led a cavalry brigade consisting of the 14th and 25th
Chasseurs à Cheval. Merlin's 6th Brigade was part of
Jean-Louis-Brigitte Espagne's Cavalry Division at the
1806 Siege of Gaeta. He became an
equerry to
Joseph Bonaparte, the new sovereign of the
Kingdom of Naples. On 1 June 1807 he took command of the division of
Salerno and
Avellino and transferred to lead the division of
Abruzzo on 9 September. Merlin was part of the kingdom's government in
Naples in May 1808 when Joseph assumed the crown of Spain. Merlin went with Joseph and became a
general of division and
captain general in
Joseph's Kingdom of Spain. At the
Battle of Talavera on 27–28 July 1809, Merlin led 1,188 men of the
IV Corps cavalry brigade. The units were the 10th and 26th Chasseurs à Cheval, Polish Lancer and Westphalian
Chevau-léger Regiments. During the battle, the British commander
Sir Arthur Wellesley ordered
William Anson's cavalry brigade to charge the French. About in front of the French defenders, the 1st Hussars of the
King's German Legion and the
23rd Light Dragoons unwittingly charged into a hidden stream bed which toppled many horses and tumbled many riders to the ground. Hastily reforming, the Germans and the left wing of the 23rd LD charged the French infantry drawn up in squares, were repulsed and withdrew. Drake's and Allen's squadrons on the right wing of the 23rd LD rode past the squares and charged Merlin's cavalry brigade. The 10th and 26th Chasseurs in the front line drew aside and allowed the British cavalry to gallop past. The two British squadrons struck the Westphalians in the second line, then were attacked from behind by the 10th and 26th Chasseurs. Only
Lord George Russell and a few other British horsemen escaped the trap. In the battle the 23rd Light Dragoons lost 207 killed, wounded and captured out of 450 horsemen. 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. After the French were driven out of Spain, Merlin received promotion to general of division in the French army on 5 January 1814. After being placed in charge of the central cavalry depot on 21 January and in command of some
National Guard units on 31 January, he assumed command of a cavalry division on 11 February. At about this time, Emperor
Napoleon reorganized his cavalry, forming the
I Cavalry Corps from the divisions of Merlin and
Étienne Tardif de Pommeroux de Bordesoulle with Bordesoulle in command of the corps. Merlin's 1st Light Cavalry Division first appeared in the order of battle during the
Battle of Champaubert on 9 February. At the
Battle of Vauchamps on 14 February, the 1st Brigade under François Isidore Wathiez consisted of the 6th, 7th and 8th Hussars and the 1st, 3rd, 5th, 7th and 8th
Chevau-léger Lancers. The 2nd Brigade under Claude Raymond Guyon was made up of the 1st, 2nd, 3rd, 6th, 8th, 9th, 16th and 25th Chasseurs à Cheval. On 20 February, Wathiez's brigade counted 1,063 sabers while Guyon's brigade had 956. The regiments were small; the largest numbered 177 and the smallest 84 horsemen. Merlin's division was involved in the fighting subsequent to the
Battle of Gué-à-Tresmes. On 2 March 1814
Friedrich von Kleist sent out a reconnaissance force of three cavalry regiments, five infantry battalions and two horse artillery batteries. Merlin charged the Prussians but was pushed back on his infantry supports. The Prussians were stopped and retreated in the late afternoon. During the
Battle of Laon on 9 March, the corps of Marshal
Auguste de Marmont and Bordesoulle were routed by a 7:30 pm Prussian surprise attack. The French lost 3,500 men and 45 guns in the so-called "Hurrah of Athies". At the
Battle of Reims on 13 March, Merlin's division advanced on the extreme right flank with the task of stopping the Prussians from falling back toward the bridge at
Sillery. The cavalrymen overran a number of Prussian battalions, compelling the soldiers to surrender. On 14 March, Merlin's troopers were beaten in a skirmish with Friedrich von Katzeler's Prussian cavalry at
Courcy, losing about 100 casualties. By 17 March, Merlin's division had shrunk to 1,150 sabers when it was assigned to Marmont's corps. When the force led by Marshals Marmont and Mortier moved east, Merlin formed its advance guard. On 23 March 1814 at
Bergères, his troopers drove off an Allied foraging party, capturing 100 men and liberating 16 wagon loads of plunder. At the
Battle of Fère-Champenoise on 25 March, Marmont and Mortier with 18,100 infantry, 4,350 cavalry and 84 guns found themselves facing 26,400 Allied cavalry and 128 guns. Merlin's division was involved in the fighting which ended in a serious French defeat. For the
Battle of Paris on 30 March Merlin's division was reduced to 850 horsemen in the brigades of Pierre François Huber and Antoine Henri Latour-Foissac. Marmont and Mortier led 19,961 foot soldiers and 5,565 horsemen against 119,000 Allied infantry and 26,500 cavalry. Bordesoulle's corps and
Louis Pierre Aimé Chastel's cavalry division were posted on the extreme right flank. After stiff fighting, Marmont and Mortier agreed to an armistice that would allow them to evacuate Paris. ==Later career==