I reiterated to Hitler my viewpoint that an invasion of England could be considered only as a last resort, in order to bring England to negotiate for peace. To achieve this last resort I told him that in my opinion the most effective weapon was a stepped-up, effective U-boat campaign, and the next most effective means would be air attacks on convoys and other important targets, such as the port of Liverpool. In contrast to the Norwegian Campaign, I could not recommend a landing in England. Aside from absolute mastery in the air, one further requisite was a dependable, absolutely mine-free zone for the troop transports. It was impossible to say how long the creation of such a mine-free channel would take, even if it could be done, or how it could then be kept free from the menace of fresh mines dropped by enemy planes. Furthermore, the flanks of the whole transport area would have to be protected by strong and effective minefields laid by ourselves.
Lastly I called attention to the face that the job of converting and readying ordinary shipping for troop transports and supply ships would be a long and tedious job, and would cause a serious stoppage in the ordinary waterborne traffic of Germany on which both our armament program and domestic economy depended. To go ahead with any real preparations before a definite decision had been made for the landing would be completely wrong. Hitler agreed. He stated that not only was mastery of the air and absolute prerequisite, but the build-up of the submarine arm was also essential.