Worldwar by Harry Turtledove (1994, Hardcover)

I first saw this book at a Super Crown book store in Schaumburg Illinois in 1995 or 1996. The cover description caught my attention as I was always an alternate history story and SciFi reader and at the time I had never heard of Harry Turtledove. I am not the most avid reader on the planet by any means, being lucky if I manage to read 4 books a year cover to cover. I started this one and finished it in less than a week, going back to see if the next was out.
I really do not want to give much of the story away, but it is basically an alternate history in which WWII was just really getting under way (I believe it was ’38 or ’39) when an alien invasion fleet arrives at Earth. Having sent a probe here in the 1200’s (or around then) and seen knights at best for our technology and weapons, they send a fleet of war ships armed with just enough to take over a planet of bumbling knights on horses, and not much more. Oh, and there is a colonization fleet running about 50 years behind the main invasion fleet, so they need to take over Earth and beat us into submission while doing a minimal amount of damage to the planet (since their people are on the way) and with a minimal amount of modern weapons and no factories to resupply what they use up. They are a bit slower than humans, in that they take time to think things out and ponder the implications of what they do (that explains the 700 years it took to go from a probe to invasion, less about 50 years traveling in space to get here).
The store revolves around the invasion fleet’s admiral, a couple of alien fighters, a couple of Americans, a couple of Germans, and couple of Russians, jumping from person to person for a chapter or 2.
Don’t expect the story to wrap up at the end. This is a series of 3 books and each is just a wonderful read.