Fighting in a series of campaigns in the Netherlands, Germany and Spain, he played an important part in the battles of Quatre Bras and Waterloo, where he was killed in action.

The following year, his regiment helped defeat the Anglo-Russian invasion of Holland, with Van Merlen fighting at Bergen, Alkmaar and Castricum.

[4] When the Batavian Republic became the Kingdom of Holland in 1806, Van Merlen found favour with King Louis and was made a Lieutenant Colonel in the Royal Guard Hussars on 23 October 1806.

However, he seems to have offended King Louis and had to leave the court, becoming colonel of the 3rd Hussars who were already serving in the Dutch Brigade and fighting in the Peninsular War.

When the Lancers departed to take part in the French invasion of Russia, Van Merlen remained at Versailles in command of the regimental depot.

[6] At the Battle of Lützen on 2 May 1813, Merlen commanded the 1st Brigade in the 3rd Light Cavalry Division under Louis Pierre Aimé Chastel.

He fought at the battle of Brienne, but two days later he was captured in a cavalry skirmish between Montierender and Wassy taking three lance thrusts to his right arm and hand.

When released at the end of the war, he learned that Napoleon had made him a Baron de l'empire on 5 April 1814, one day before his abdication.

[8] When Napoleon returned from Elba in 1815, Van Merlen remained loyal to his new master and, like many other Dutch and Belgian officers, prepared to go into battle against an army in which he had served all his life.

[11] The brigade was heavily engaged at Quatre Bras, and held in reserve at Waterloo, where they helped drive off the French cavalry attack.