A Spanish victory in the Armada. A world where Bolshevism didn't succeed in Russia. A world where nuclear physics didn't get off the ground, and we've already fought a conventional WW3. It seems to me that one of the most difficult parts of writing alternate history would be extrapolating the results of a single change, while keeping your story believable, especially considering how many improbable and unbelievable events have occurred in real history. This is certainly true.
All history has to do is happen.
Fiction has to be plausible, too. If anyone wrote a novel in 1999 setting out the 2000 election in detail, no editor would have bought it because it would have seemed too unlikely. The point is, if the despatch hadn't been lost, the battle would have happened somewhere else, and under different conditions, which is the key to the breakpoint. Changes should be made at places that are both interesting and would have important consequences if they happened differently. You re a frequent guest at science fiction conventions, including your turn as Toastmaster at last year s WorldCon in Chicago.