According to Davidson, Cogny was in fact the officer who proposed Dien Bien Phu as a "mooring point" to Navarre. Jules Roy, however, disagrees. Where Cogny had envisioned a light mobile base of operations, Roy argues, Navarre saw a heavily defended fortress. Cogny was one of many officers who protested against this new strategy, stating that "we are running the risk of a new Na San under worse conditions" (Na San was successfully defended during a nine-day siege but was thereafter evacuated). These protests, however, had no effect. Though stationed in Hanoi during the Battle of Dien Bien Phu itself, having seen the battle turning against France, Cogny attempted to reach the besieged garrison to take command; however, his aircraft was beaten back by anti-air fire on 17 March 1954. He considered parachuting in, but was advised against it. Throughout the battle, Cogny and his superior Navarre were at odds on the disposition of forces between Dien Bien Phu, Cogny's own sector in the Tonkin Delta, and Navarre's Operation 'Atlante'. 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. The time scale for his departure was not discussed at the time, and Cogny continued to serve under Navarre, with the relationship between the two degrading significantly. 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. On 2 May, Navarre went as far to threaten Cogny with an investigation into his 'defeatist' press releases. As Dien Bien Phu was about to fall, it was Cogny who took the final radio calls from the commander of the garrison there, Colonel Christian de Castries.