Ecole Spéciale Militaire de Saint-Cyr
Born on 16 February 1858 at Bouvines, Deffontaines was from a family of farmers from the region. He joined the Ecole Spéciale Militaire de Saint-Cyr as an officer cadet on 30 October 1878. He was promoted to cadet first class on 22 December 1879 and to corporal on 21 March 1880. His graduation class was named for the Zulu race, which had recently fought in the Anglo-Zulu War, and he was commissioned into the 28th Infantry Regiment as a "sous lieutenant" (second lieutenant) on 10 October 1880.