Second Variety by Philip K. Dick, Science Fiction, Fantasy

Across the ground something small and metallic came, flashing in the dull sunlight of mid-day. A metal sphere. It raced up the hill after the Russian, its treads flying. It was small, one of the baby ones. Its claws were out, two razor projections spinning in a blur of white steel. The Russian heard it. He turned instantly, firing. The sphere dissolved into particles. But already a second had emerged and was following the first. The Russian fired again. A third sphere leaped up the Russian's leg, clicking and whirring. It jumped to the shoulder. The spinning blades disappeared into the Russian's throat. Eric relaxed. "Well, that's that. God, those things give me the creeps." The claws were bad enough in the first place -- nasty, crawling little death-robots. But when they began to imitate their creators, it was time for the human race to make peace -- if it could!