Benefit from the Fighting between Mussel and Snipe
yu (4) bang (4) xiang (1) zheng (1), yu (2)weng (1) de (2) li (4)
Seven states were locked in battle at the end of the Warring States Period (220-280 AD). The Duke Hui of the State of Zhao wanted to conquer the State of Yan. Real bad. That is, until he talked to Su Dai, one of those advisers.
Su said: "One fine day, a mussel went out to the beach to sunbathe. Meanwhile a snipe (which is a kind of bird that looks like a sandpiper with a long bill) caught sight of the tasty tidbit. He plunged to bite the meat. But the mussel immediately closed up, meaning the snipe could not pull out its beak out of the shell. Both of them became locked in mortal combat. Along came a fisherman and bagged them both.
"If you go to conquer Yan now, Zhao and Yan will be like the mussel and the snipe. Meanwhile the strong Qin State will be make like the fisherman and gobble up both territories."
And so Duke Hui never ate seafood again. Just kidding. The idiom of course refers to a mutually-destructive battle between two parties, especially one involving unshakeable principles, thus leaving the coast clear for a third party to swoop in late and sweep up the spoils.