Short story where advanced civilization doesn't understand their technology

I've been trying to find a Ray Bradbury short story I read a number of years ago. The details are a little fuzzy in my mind, which is why I want to find it, but the gist is this:

Some explorers come across a civilization with highly advanced technology, but none of its users understand how it works. I believe the idea was the technology was built by machines, which were built by other machines, which were built by their ancestors long dead. This dependency on technology they don't understand makes the explorers think this civilization will be easy to conquer, but they are mistaken.

Any ideas?