The "Settling Accounts" series (my name) was pure alternate history. A big what if. This is a big what if too, but it mixes "alternate" history with science fiction. In the "Colonization Series" the planet is happily involved in WW II when aliens invade, intent on conquering everybody, and humanity has to unite (uneasily) to fight the invaders.
For the sake of clarity, time will be expressed in earth units. 24 hrs to a day, 365 days to a year and so on. The alien units were different.
The aliens were a reptilian based life form, walked more or less upright, and grew to about the size of an average 10 year old human. Humans called them "Lizards", and they called humans "Big Uglies".
The Lizards sent a probe to earth in the 12th century, gathered all kinds of information about the climate and civilizations, and decided it was a very good candidate to conquer and add to their empire. They had conquered two other planets a few thousand years earlier, and these planets had assimulated nicely. However these planets were also inhabited by reptilian based life forms, and were a pre-industrial civilization when they were conquered.
The Lizards were a very patient civilization. They'd been unified for 50,000 years or so, and change came very slowly to them. They had no idea that human beings were any different.
So imagine the shock of the fleetlord of the invasion fleet, when he entered earth orbit a mere 800 years later, and found not knights in rusty armor, fighting with swords, but a highly industrial society, with flying machines and powerful weapons, on the verge of discovering atomic energy, and engaged in a planet wide war amongst themselves.
This was NOT the way it was supposed to be. And uncharacteristically, the fleetlord had a decision to make. It took the Lizard invasion fleet 20 years to get from "Home" to Earth. It would take another 20 years to get a message back to Home, and then another 20 years to receive a reply. The colonization fleet was already on the way, less than 20 years behind, and they expected to find a conquered planet when they arrived. It was too late for them to turn around. So, the fleetlord, with full knowledge that the planners of the invasion had been wrong, attacked.
The Lizards did nothing without careful planning. They were the ultimate Capability Maturity Model or ISO Certified civilization. Nothing new was introduced into society without careful study, sometimes for centuries, to make sure it would not have undesired effects. Change was so slow, that a Lizard might not even notice it in a lifetime. And that was saying something, because they lived 130 years or so. So for the invasion fleet to come upon something so completely unexpected was something that no Lizard had ever experienced. "Home" wouldn't even realize there was a problem until 40 years after the invasion fleet left. They were on their own, and had to make a decision with incomplete knowledge of the situation, something no Lizard had had to do for centuries. From the very beginning, things were going wrong for the Lizards.
The Lizards had come loaded for bear anyway, even though they expected to be fighting primitives. They were far advanced of humans in space flight, obviously, but in their weapon technology, they weren't as far ahead as it first appeared. They had jets, but humans were just a few year away from jet fighters, just a few years away from atomic weapons and so on. Human tanks weren't as good as the Lizard equivalent, but human tactics were much better. They'd had a lot more practice. The Lizards showed up on earth 1942, with weapons of the year 2000 or so.
In many areas of earth the Lizards found primitive societies, and they conquered them without too much problem. But in North America, Europe and parts of Asia, the Lizards had extreme problems.
Eventully, the war ended. The North Americans, Europeans (except Poland) and Japan retained their independence. China was nominally controlled by the Lizards, but there was wide spread unrest amongst the population, to put it mildly.
So the wars stopped, the dust settled, the colonization fleet approached, and Lizards and humans began to interact.
And that's what it is. 11 volumes of humans interacting with an advanced alien species, buying borrowing and stealing their technology, until the time comes, about 85 years later, when humans are actually more advanced than the Lizards.
The last book in the series, "Homeward Bound", is pure science fiction. Humans make it to "Home", and the Lizards are not happy about it. The ultimate "certified" society had goofed. They found themselves without the flexibility and without the ability to adapt to the rapid change humans were accustomed to. The "almost" conquerors are very close to being the conquered.
I enjoyed reading these Harry Turtledove books. But I think it'll be awhile before I pick up another one. I estimate I've read between 10,000 - 11,000 pages of Harry Turtledove in the last year, and that's enough for awhile.
Time for a yard sale.