The military base at Mailly was just north of the community of Mailly-le-Camp, 80 miles east of Paris along the national road linking Châlons-sur-Marne to Troyes. It was built in 1902 as a barracks and training ground for the French army. In the 1930s it was used for their armoured formations, but since the fall of France it had been taken over by the Germans and used to train replacement crews for panzer units refitting from losses suffered in the east. In the autumn of 1943 requests to monitor the installation were sent to l'Armée Secrète. The organization's Brutus-Nord network was given the assignment of producing detailed plans of the panzer training grounds, and for providing a regularly updated inventory of the German units present.`In the spring of 1944 the group reported the arrival of elements of the 21st Panzer Division.