As I mentioned before, this summer I finished Harry Turtledove's massive 11-volume The Great War series. He is the king of alternate history, and this series postulates that the South won the Civil War (or the War between the States, as my redneck relatives call it) with the help of France and England, who actually were considering weighing in on the South's side in real time. Leading out from that is a very interesting set of novels that question what political alliances/technology advances/governments would naturally procede from that new situation. The United States (the North) becomes allies with Germany, which when World War I comes around, creates a battlefront across the entire North American continent in two places, as the US is attacked by the Confederate States and Canada. Turtledove even creates a Hitleresque figure in the South that hates blacks as much as his real time figure hated Jews. With the help of a few embittered compatriots, he even devises "concentration camps" for them, and nearly wipes out the black population of the South.
All of that to say that I am re-reading another of his series, The Worldwar Saga. This begins in the middle of WWII, and asks the question, if the nations of earth at that point were faced with an outside enemy, would they be able to lay aside their differences and put fighting for the survival of the human race above their political ideals?
These books are not for the casual reader -- they will be lost in a sea of characters and details. But if you could keep track of where Frodo, Merry, Pippin and Aragon were in The Two Towers, dig in with relish. Real people and circumstances are sprinkled throughout these series like happy surprises. I have finished Worldwar: In the Balance, Worldwar: Tilting the Balance, and am halfway through Worldwar: Upsetting the Balance. And on a cheer for the home team, Decatur, IL is mentioned again and again as one character had been a minor league player for the Decatur Commodores! Yay for the midwest!