« Homeward Bound » Harry Turtledove

The twentieth century was awash in war. World powers were pouring men and machines onto the killing fields of Europe. Then, in one dramatic stroke, a divided planet was changed forever. An alien race attacked Earth, and for every nation, every human being, new battle lines were drawn …

Now–in the twenty–first century–a few daring men and women embark upon a journey no human has made before. Warriors, diplomats, traitors, and exiles–the humans who arrive in the place called Home find themselves genuine strangers on a strange world, and at the center of a flash point with terrifying potential. For their arrival on the alien home world may drive the enemy to make the ultimate decision–to annihilate an entire planet, rather than allow the human contagion to spread. It may be that nothing can deter them from this course.