Montfaucon served in the French army as a volunteer and participated in the Franco-Dutch War of 1673. He was a captain of grenadiers and made two campaigns under the command of Marshall Turenne, participated in the Battle of Herbsthausen and fell ill in Saverne in Alsace. Because of his infectious illness he made a vow to Our Lady of Marceille to give one hundred "livres" to her sanctuary in Limoux and to become a monk, if he was able to return to his country as a result of her intervention.