From the moment the invaders arrived, breathed our air, ate and drank, they were doomed. They were undone, destroyed, after all of man's weapons and devices had failed, by the tiniest creatures that God, in His wisdom, put upon this Earth. By the toll of a billion deaths, man had earned his immunity, his right to survive among this planet's infinite organisms. And that right is ours against all challenges, for neither do men live, nor die, in vain.
The last lines of the movie the Narrator explains that from the moment they arrived they were doomed. He explains that while man had earned his immunity, the aliens had died due to the tinest creatures man had put on earth, referring to bacteria or viruses. The final line is that men do not live nor die in vain.