If the results of air warfare are unsatisfactory invasion preparations will be stopped.
First of all, success is to be achieved within the target areas of Luftflotten 2 and 3. Only a complete victory over the RAF in southern England can give us the possibility of further attacks on enemy forces stationed in the depth of the country.
[Hitler] admitted that the invasion of England would be an extremely daring enterprise. He agreed that we could not expect to effect a surprise landing, and that we would be facing a most resolute opponent who controlled the water area we had to cross. Also he conceded that to keep the invasion force of approximately 40 divisions supplied after they had landed would present the utmost difficulty. An absolute prerequisite for the whole operation would be the complete supremacy in the air and an effective minefield protection along both flanks of the crossing channel, plus the added protection of a barrage from heavy guns installed on the French side of the Straights of Dover. He concluded by saying that owing to the lateness of the seasons, the main operation would have to be completed by 16 September, and therefore if all preparations had not been completed in time for the landing to begin by the first of September, then other plans would have to be considered.