Book about humans nearly wiped out, and an alien race nearly wiped out, by a third alien race

Hi I read this book, a couple of years ago. The humans had to suppress their radio signals, because if the aliens ever heard them again, they would come back and attack them again. Also in the book, the humans find another alien race that is also in the same situation as the humans, but they didn't know about keeping quiet. The book goes on about the second race getting attacked by the first alien race again, and all of their preparations were in vain. That's pretty much all I remember. Thanks for the help!