Phillip K. Dick short story about paranoid AI

This is a short story a friend of mine told me about some time ago.

It was written by Philip K. Dick. It was about an AI which controlled US defense system The AI decided that the main threat for humanity was something ridiculous (I don't remember what exactly it was). Because of this, the government/scientists concluded that the AI was broken and decided to turn the machine down.

The plot twist at the end was that