He was a commander of the French forces in Tonkin (northern Vietnam) during the First Indochina War, and notably during the Battle of Dien Bien Phu.
An academically gifted boy, Cogny was awarded a scholarship to École Polytechnique, where he received an engineering degree, a diploma from the French Institute of Political Science,[3][4] and a doctorate in law.
He was held in captivity for almost a year before he escaped in May by crawling naked through a drain pipe with three companions, pushing their disguises out in front of them.
[7] In 1943, now a major, he was arrested by the Gestapo and underwent six months of interrogation and torture in Fresnes prison before being sent to Buchenwald and, later, Mauthausen, concentration camps.
He was sent with de Lattre to French Indochina in 1950 and after the latter's departure commanded a division in Tonkin and a Groupe Mobile in the Red River Delta.
[3] According to Davidson, Cogny was in fact the officer who proposed Dien Bien Phu as a "mooring point"[8] to Navarre.
Where Cogny had envisioned a light mobile base of operations, Roy argues, Navarre saw a heavily defended fortress.
In response to a damning letter from Navarre on 29 March, Cogny informed his superior that he no longer wished to serve under his command.
Cogny would continue attempts to with-hold certain reinforcements from Dien Bien Phu or relating relief efforts if he believed it would undermine his strength in the Tonkin Delta.
[13][14] As Dien Bien Phu was about to fall, it was Cogny who took the final radio calls from the commander of the garrison there,[3] Colonel Christian de Castries.
(literally 'my old one', more commonly translated as 'my friend', 'my fellow' or the phrase 'old man' in a sense of friendship)[3] By nightfall, all French central positions had been captured and Dien Bien Phu had fallen.