Worldwar
Worldwar is a series of four alternate history novels by Harry Turtledove. The basic premise is that aliens invade Earth right in the middle of World War II. Although the aliens, a basically reptilian species, have far superior technology, their last information on humans was collected by a robot probe 800 years before, and the "Lizards" find themselves facing much tougher opposition than the knights on horseback that they expected. The narrative follows the intersecting fortunes of a large number of human and alien characters. A follow-on trilogy, Colonization carries the story forward into a very different 1960s.The books are a fun read, if not great literature. Turtledove is better at working out the entertaining consequences of the premise than at developing his characters; both the humans and Lizards are predictable, and the conversations wooden (ironically, this is part of the Lizards' culture, so they are not being that poorly represented!). In the end, though, it is really the changed world itself that is the star of the show.
The volumes are:
