I read this story in a collection of short stories the 1980's and I am now looking for the publication. All I can recall is that it was about a warrior race imprisoned on their own planet by an Alliance of other planets. This imprisonment took the form of placing a force field around the entire planet that was controlled by the planet's moon. The Alliance does not contact the warrior race for years; but eventually sends a delegation to the planet to ask for their help defending against an invader. The Alliance finds what appears to be a peaceful agrarian society with little or no appearance of a warrior race. Most of the Alliance delegation thinks there is little that the former Warrior race can do to help so they agree to turn off the moon force shield and the former Warrior race agrees to come to their aid. In the mean time an away party, not the negotiation party finds out that the entire planet is filled with machinery, but they are unable to communicate this to the negotiation party before they leave.
At the end of the story the Alliance ask will the Warrior race help since they do not appear to have any ships; the story ends with the entire planet being revealed as a ship and the formal imprisoning moon being drawn along with the planet ship, once the force field generator is destroyed. Can anyone name this story or help me find it???