As Asimov points out, the secret directive to set up what would become the Manhattan Project came up for President Franklin D. Roosevelt's signature on Saturday, December 6, 1941 - and he did sign it. However, it being near the weekend, Roosevelt might easily have felt tired and postponed this particular signature until the next Monday - and in that case, since the Japanese surprise attack on Pearl Harbor happened on the following morning, Roosevelt might have been so busy as to forget all about it, in which case the Manhattan Project would never have come into being.