The gist of it: Price Albert passes away sooner than he did and as a result is unable to prevent the British from going to war with the Union. Now, that in itself would make for a very short book as the Union would collapse being attacked from both ends. However, let's just say that something goes wrong with the British's surprise attack.
TS
It is a very good series, as is the Turtledove series "the Great War." And if you haven't read his "Guns of the South," you better.
I have read the first two and they are great. I will now have to look for the third.