At the Berghof conference on 31 July, the "Luftwaffe" were not represented. Raeder said barge conversions would take until 15 September, leaving the only possible 1940 invasion dates as 22–26 September, when weather was likely to be unsuitable. Landings would have to be on a narrow front, and would be better in spring 1941. Hitler wanted the invasion in September as the British army was increasing in strength. After Raeder left, Hitler told von Brauchitsch and Halder that the air attack was to start around 5 August; eight to fourteen days after that, he would decide on the landing operation. London was showing new-found optimism, and he attributed this to their hopes of intervention by Russia, which Germany was to attack in the spring of 1941.