In the 1930s, France and the U.S.S.R. signed the Treaty of Mutual Assistance that opened up, among other things, the ability to purchase military equipment from each other. On a Soviet order, Schneider developed a 406 mm gun that was planned to be mounted on Soviet battleships. The outbreak of World War II slowed down the construction of battleships in the U.S.S.R., and these guns could well have been used on battleships under construction in France. To fit the barbette of a 380 mm quadruple-gun turret, the 406 mm turret had to be made a triple one.