Not a single surplus existed for extensive experimentation."[3] The design work for what became the Bismarck class was begun in 1933 and continued until 1936.[4] During this period, the naval leadership grappled with a variety of problems, including the strategic and operational use of such a ship and the preferred propulsion system. War games suggested that the new battleships could be used both to attack French shipping at long range and to fight in a traditional battle against the French and Polish navies, then seen as the most likely threats.[5] Due to the numerical inferiority of the German fleet and the assumption that naval battles would take place at relatively close range in the North Sea, the design staff placed great emphasis on stability and armor protection. Very thick vertical belt armor was adopted, along with heavy upper-citadel armor plating and extensive splinter protection in the bow and stern of the ships.[6]