On 2 July, the OKW asked the services to start preliminary planning for an invasion, as Hitler had concluded that invasion would be achievable in certain conditions, the first of which was command of the air, and specifically asked the "Luftwaffe" when this would be achieved. On 4 July, after asking General Erich Marcks to begin planning an attack on Russia, Halder heard from the "Luftwaffe" that they planned to eliminate the RAF, destroying its aircraft manufacturing and supply systems, with damage to naval forces as a secondary aim. A "Luftwaffe" report presented to the OKW at a meeting on 11 July said that it would take 14 to 28 days to achieve air superiority. The meeting also heard that England was discussing an agreement with Russia. On the same day, Grand Admiral Raeder visited Hitler at the Berghof to persuade him that the best way to pressure the British into a peace agreement would be a siege combining air and submarine attacks. Hitler agreed with him that invasion would be a last resort.