I am currently reading the first book in an alternate history trilogy by Peter G. Tsouras. Britannia's Fist imagines a British entry into the American Civil War, specifically against the Union.
Tsouras is a retired US Army intelligence officer. Although I've heard of his name before, I have yet to read his work. So far, several pages in, it is clear that Tsouras is trying to create a historically plausible scenario. This is the most important task in the writing of alternate history fiction.
Harry Harrison wrote a similar trilogy, beginning with The Stars and Stripes Forever. I would describe it as wildly optimistic because every lucky break goes America's way. Nonetheless, it is fairly plausible and worth reading.