The story is about a giant alien snail who monitors a region of space waiting for the Ancients to awake - and they're not good. When the Ancients awake the "snails" seek help from humans. Earth decides to help and bring their moon with them.
Been a few years, but is sounds like "With Friends Like These" by Alan Dean Foster. Aliens come to Earth looking for help against some enemy and discover the humans living as simple farmers. However, they live that way by choice and are actually so advanced they prefer to live a simple agrarian lifestyle. In the end the aliens draft them into their cause and the humans move Earth and the Moon to follow the aliens.