This book is a continuation of the American Empire novels, which is a continuation of The Great War novels, which goes back even further. Depending on how you count, this book is either the 7th or 8th in the series (How Few Remain can be considered the beginning of this series).
The year is about 1940. The Confederate States of America (CSA) successfully broke away from the US back in the 1860s and have had numerous wars with the US in the meantime. In a lot of ways, imagine that World War 1 took place with the US as the central theater instead of Europe. Same with this, only it’s now what we would consider World War 2. I don’t have a great history of World War 1 or 2, but you can see a lot of similarities. The CSA (Germany) had their economy destroyed as a result of the cease fire by their stronger opponents (the rest of Europe). A charismatic Jake Featherson (Adolf Hitler) tells the population that their ills are caused by the terms of the previous war and he’s not going to take it anymore.
The world Turtledove has created shows how small changes in history can have larger impacts later on. FDR makes an appearance as an assistance secretary of war, his chances of being US President ruined once word of his Polio was known. MacArthur and Patton are tank commanders facing off outside Richmond VA. Quebec is an independent state and the remainder of Canada has been occupied by US and Quebecois forces for some 30 years. Mormons in Utah have decided they wanted independence as well, even though their state is surrounded by the US. They have waged a guerrilla war with funding by the CSA.
As usual, Turtledove shows the world through the eyes of not one, not two, but close to 20 different people from all over the map. Since his story has gone on for almost 80 years, older generations die off of old age or war and the story picks up with their offspring. The scenes change rapidly between people, so it’s sometimes hard to (say) follow the explits of Sam Carsten rounding Cape Horn then immediately switch to Jefferson Pinkard running a ‘camp’ and wondering how to ‘reduce the population’ of its inmates.
If you’re interested in alternate history, I’d recommend one of Turtledove’s first books in the genre: Guns Of the South. It’s alternative history with a touch of sci-fi to it.