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A group of aliens come to earth in desperation as their federation of worlds is under attack by a powerful invading species. In the distant past the earthlings were a powerful race that was spreading out too fast so the aliens pushed them back and placed force field around earth to contain the violent earthlings. The aliens are disappointed as first as it seems that the earthlings have devolved to a simple farming planet. Later it is revealed that the simple life is a ruse and that the humans have developed to an insane and powerful degree. They agree to effortlessly destroy the invaders but now the delegation wonders what will happen to them.

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This is Alan Dean Foster's "With Friends Like These..." (1971), first published in Analog, but you probably read it in the same-named collection.

The powerful invading species are the Yop who will eat anything - and anyone - and so have no allies, but there are a lot of them.

The earlier powerful race were the Veen ("Up the Veen!") who went to war with Earth when their predictions suggested the alternative was to get ready for human rule. After finally pushing humanity back to Earth they erected the Shield on the Moon which can't be taken down from inside. (It was a pyrrhic victory though, "there are no more Veen.")

The aliens drop the Shield ("The Shield is down!") before one of their scouts returns to reveal that the entire planet is full of automated factories, and that he saw a boy destroy an entire Yop battlewagon. ("More bad mans.")

The end of the story has Earth follow the aliens to do battle - bringing the Moon along - and one of the aliens wondering to another "What will we do when there are no more Yops?"

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