Well, volume 4 of the Great War Tetralogy turned out to be volume 1 of the American Empire series. This book starts to deal with the inter war years and how the USA and CSA cope with having fought such a destructive war. Unsurprisingly, they react similarily to Europe in the inter war years and this, I feel, is part of what Turtledove is trying to show, that the circumstances of history mould countries. The CSA and USA could be Germany and France and their histories are similar.
Obviously, this is the more shocking with the CSA. Whilst the sight of the USA gaining its first ever Socialist government is moderately amusing and interesting, the slow descent of the CSA into fascism is shocking and disturbing. Previous books have shown Communism as an American idea and this book shows Fascism as an American idea drawing on the only too familiar roots of Racism that were around in the real 1920s Southern States (and, lets be honest, weren't too far away in the North either.) And I wonder where Turtledove is going with this, though I think I can guess. With Fascism and Communism being American ideas, it seems only too likely that the Holocaust will be an American idea also, with a Black face rather than a Jewish one.
I could criticise this book, for example the history of the CSA seems to paralell inter-war Germany a little too closely with Donald Featherstone's Freedom Party paralleling the Nazis too much. But I would still reccomend it. The story seems to get to 1924-25 (Turtledove's a little hazy about dates) so there's plenty of time to go before WWII - although it doesn't necessarily have to start in 1939, I guess, so maybe there's less time than it seems.