When the Kingdom of Holland was annexed by France in July 1810, Van Merlen found himself once more in French service. On 11 November 1810 he was transferred to the 2nd Regiment of Lancers in the French Imperial Guard, the "Red Lancers", which was formed mainly for former members of the Dutch Royal Guard. He remained serving in Spain, and it was not until 11 March 1812 that his remaining hussars formally became the fifth squadron of the Red Lancers, with him as their colonel. When the Lancers departed to take part in the French invasion of Russia, Van Merlen remained at Versailles in command of the regimental depot. Here, he played a part in suppressing the Malet coup of 1812.