Second Variety by Philip K. Dick

The story takes place in in the post-nuclear landscape of Russia, who is at war with the United Nations. The UN has developed a series of crab-like robots, called claws, that destroy human targets. The story opens with a Russian approaching the UN bunker and getting completely obliterated by several claws. The UN forces realize that the soldier was carrying a distress signal and they send Hendricks to negotiate. Along the way, Hendricks meets a small boy, David. David refuses to stay back and Hendricks allows him to follow.

When they reach the meeting place, three Russians fire at the two travels, destroying the boy. But to Hendrick's surprise the boy was a robot. The three Russians- Klaus, Ruddi, and Tasso, a local prostitute- explain that the claws have developed their own killer robots, which have annihilated their forces. David is the third variety, or III-V. A wounded soldier was the first, or I-V. That means that they have yet to identify the second variety, II-V. During the night, Klaus kills Rudi, believing he is the second kind, but he was not.

The three remaining men make their way back to the UN camp, each suspicious of the next. When they reach it, there is no sign of life, just several David's attack them. In the gunfight, Tasso kills Klaus, gears and wheels go flying, meaning he must be the II-V. Tasso also throws a grenade-like bomb that destroys all the David's.
Suffering from internal injuries and a wounded arm, Hendrick's hopes to escape to the secret UN moon base. He and Tasso find the hidden escape pod, but it holds only one passenger. Tasso convinces Hendricks to let her take the pod and return for help. She flies away and Hendricks looks at Klaus' remains, which reveal he was the IV-V robot, not the II-V. He is attacked by a group of robots, including the wounded soldier, David, and several Tassos. Hendrick does have one final comfort as he sees that the Tassos have the bombs, which means the robots are already building machines that will destroy their own kind.

 Dick said of the story: "My grand theme—who is human and who only appears (masquerading) as human?—emerges most fully. Unless we can individually and collectively be certain of the answer to this question, we face what is, in my view, the most serious problem possible. Without answering it adequately, we cannot even be certain of our own selves. I cannot even know myself, let alone you. So I keep working on this theme; to me nothing is as important a question. And the answer comes very hard."

An influential science fiction story from Dick. Great ironic suspense tale.