Poul Anderson, After Doomsday (1962)
The crew of the spaceship Franklin are captured by a nomadic society called the Kandemirians, who are desperate for the Americans' superior technology. They threaten to kill the crew unless the captain agrees to reveal technological secrets. Donnan, one of the prisoners who is coerced into sharing this technology, pretends to comply, but actually makes guns instead. In a scene bespeaking American "idealized self-regard," the American crew fights off the Kandemirians and escapes. I won't reveal "who murdered Earth"--you'll have to read the novel yourself to find out!