John Barnes wrote a whole series of this kind of alt-history (e.g. "Caesar's Bicycle", "Washington's Dirigible", etc.). Which reminds me that there is, of cour

John Barnes wrote a whole series of this kind of alt-history (e.g. "Caesar's Bicycle", "Washington's Dirigible", etc.). Which reminds me that there is, of course, a entire genre called "alt-history", starting presumably with Twain's "A Connecticut Yankee in King Arthur's Court", and notably including L. Sprague deCamp's "Lest Darkness Fall" (1941, and online in its entirety at scribd by arrangement with the author), and branching out a lot over the past decade or so.