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. Three days later he was marching with the Dutch army into Germany to support French forces in the war with Prussia. He saw service near Ertzen on 7 November 1806 and drove the Prussian vanguard into Hamelin. However, a dispute arose between Louis and his brother Napoleon which resulted in the Royal Guard being returned to The Hague without seeing further action.