The four book "In the Balance" is just that, an alternative history of WW2 if earth had been attacked by aliens. A great book with good use of real historical figures.
I especially loved Turtledoves "Guns of the South."
I think the series is up to eight books now. I have the most recent waiting for me at home, I just have to finish "Blog" first before I start on it.
I first stumbled across "Guns Of The South" when it was being read on the NPR show "Radio Reader." I had stopped the car and my family had gotten out to go get McDonalds and I sat there listening. They were well into the book and describing a particular Civil War battle that I recognized, the Battle Of the Wilderness. I was impressed with the accurate description that allowed me to recognize it without the author actually saying "this is the battle of the wilderness." I had no idea that this was an alternate history novel, let alone SF.
The action was taking place near the crossroads and the rebel forces were running out of ammunition. Yep, that happened. And finally the ammunition wagons pulled up, but the first few crates had the wrong ammunition, all mini-balls. O.K., I thought the rebels were still using mini-balls at this stage and that was one of the reasons they lost the battle, but I'd go along. Oh, they found another case with the right ammunition... the banana clips...
BANANA CLIP??????????
.... for their AK-47s.....
AK-47S!!!!!!!!!!!!!!!!!!!!!!!!
That's the point I knew I had to buy the book and find out what this was all about. The thing that so impressed me also impressed Shelby Foote, the great Civil War historian. He thought so much of the accuracy of the manuscript (except for the things directly changed by the time travler plot element) that he wrote the forward for the book.
That "little" left turn that Turtledove used to allow the south to win was a bit too much, even for his tastes. He has revisited the idea of the south winning the war in another series of books, but with a much more subtle change, where battle plans lost by a southern general (in reality) and recovered by the north, leading to a southern defeat, are recovered instead by a southern soldier (in his story), thus altering the whole history of the war.
I got bored with the World At War books after it became apparent that book 3 wasn't going to resolve anything and haven't gone back to them yet. Someday I may.