the problems caused by the need to begin a campaign in poland in late august or early september in order to have the campaign finished before the october rains arrived , and the need to have sufficient time to concentrate german troops on the polish border left hitler in a self-imposed situation in august 1939 where soviet co-operation was absolutely crucial if he were to have a war that year and , on 23 august 1939 , joseph stalin accepted hitler 's proposal to conclude a non-aggression pact , whose secret protocols contained an agreement to partition poland .