The submarine pens and other Atlantic Wall fortifications had been built in 1940 and 1941, when the Germans had air superiority and could deter Allied air attacks. By 1942 this advantage had been lost to the United States Army Air Forces, which had begun deploying to England in May 1942, and a greatly expanded Royal Air Force. The German Army preferred an alternative approach which would use trailer-style mobile launch platforms called "Meillerwagen" accompanied by testing and fuelling equipment mounted on railway cars or trucks. Although this configuration was far less efficient and would have a much lower rate of fire, it would have the great advantage of presenting a much smaller target for the Allied air forces. The Army was not convinced that fixed bunkers could resist repeated air attacks and was particularly concerned about the vulnerability of the launch sites' road and rail links, which were essential for resupplying them with missiles and fuel.