Homeward Bound — Harry Turtledove

In the twenty-first century, Earth is teeming with hostile aliens known as the Race. With geopolitical relationships boldly shifting into unheard-of alliances and mankind's technology advancing at a dangerously accelerated pace, a few daring American men and women embark upon a journey no human has made before--traveling to the aliens' home planet. Once there, the warriors, diplomats, traitors, and exiles find themselves strangers in a strange world, and on the cusp of a flash point with terrifying potential. For their arrival 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 catastrophic course.