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Back in the '80s I read a short story (which might have been old, then) about aliens in a war with other aliens finding Earth which had previously been locked away, I think, by some kind of lock on the Moon. After some misgivings, they unlocked Earth and landed to ask for help. Earth seemed completely pastoral and they almost gave up hope of getting help, but persisted and humans finally agreed to help them. At the end of the story, they get back in their space ship and wait for Earth's space ships to fly out and help them when the Earth itself grows an engine and flies after them.
I have been looking for this story on and off for a couple of decades and have never found it, but my daughter was asking about some of the short stories I enjoyed as a youngster and this one came up, so I'd really like to find it, now. I've looked through several hundred old anthologies and haven't found it, yet, but I found StackExchange, so I'm hoping someone here can help me. I know this isn't much detail to work with, but I'm working from old memories. Thanks in advance for the help (or attempt).