De Cortmontaigne joined the French army in 1713, participating in the Rhineland campaign, the final part of the War of the Spanish Succession before passing the exam for the Corps royal du génie in 1715.

He became ingénieur en chef or chief engineer at Metz, constructing double crownworks at Bellecroix, Fort Moselle and Yutz, which he claimed to be adaptations of Vauban's original designs and in 1745, he was appointed Director of Fortifications for the Three Bishoprics.

Although viewed as a competent engineer, his prominence has been attributed to the ability to write clear and concise memoranda, with a wealth of detail; during a debate in 1741 with Forest de Bélidor over upgrades to the defences of Metz and Neuf-Brisach, he 'proved' his designs superior, using calculations he claimed were based on Vauban's principles.

[6] Asfeld managed the engineering corps like an extended family; over the years, ability became less important than connections and after his death in 1743, the Comte d'Argenson, Minister of War, sought to impose a more structured approach.

As far back as 1701, Marlborough had argued winning one battle was more beneficial than taking 12 fortresses; this line was followed by many others, notably Marshall Saxe, the most successful French general of the period, whose work on the art of war, Mes Rêveries, was published posthumously in 1757.