The Ifs of History is an introductory essay to Harry Turtledove's Agent of Byzantium, written by Isaac Asimov. Before going on to refer to that book's main premise - i.e. Muhammad converting to Christianity and as a result the Byzantine Empire surviving with its full power and territorial extent - Asimov discusses alternate history in general and sets out in particular a specific historic event which could be used as the basis of an alternate history story or book.
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.
Asimov concludes: "What would have happned? Would Germany have gotten the bomb first? Would World War II have ended without the bomb and would the Soviet Union have gotten it first? Would no one ever have developed the bomb? You could write three diffferent stories about three different consequences from this one little if of history - if Roosevelt had yawned and said I'll do it Monday".
In practice, neither Asimov himself nor Turtledove nor anybody else is known to have written any story based on this specific Point of departure.