Collision Course — Robert Silverberg

After four centuries of space travel Man was supreme. human civilization had spread to hundreds of planets, and never, *never* had the colonists found a trace of another intelligent species. Humanity was alone, with the entire galaxy at their command.

And then the unthinkable happened. An experimental ship, probing far beyond Earth's most distant outposts, discovered a new colony -- and the colonists were not human.

An alien empire was advancing inexorably on Earth. Imperial Norgla believed they were alone in the galaxy, and that all the stars were *theirs* to claim. Earth and Norgla would have to compromise -- or both would be destroyed.