Short story by Bob Shaw, first published in New Worlds issue 155 (1965). Shaw's first professional publication.

For five years, alien ships have been landing on Earth, one every twenty-two hours, spewing out hordes of scaly black, disease-ridden monsters. For five years, the United Nations Planetary Defence Force has been repelling them, wherever they land. It has been discovered that the invaders are merely colonists whose nature is so alien to ours, that co-existence is impossible. Yet the caravan of ships stretches right out beyond the edge of the solar system, so there is no sign of the invasion stopping any time soon.

Lt. Col. John Fortune is a former war hero, but now languishes in an out-of-the-way posting in Iceland. Before their lack of hostile intentions was known, Fortune was one of the first soldiers to discover a method of effectively killing the aliens. He is now wracked with guilt over having participated in the slaughter of innocent homesteaders. During the story, he redeems himself by finding a way divert the alien caravan and halt the killing.