Like a Pyrrhic victory, a Cadmean victory comes at as great a loss to the winners as to the vanquished.
Cadmean derives from Cadmus, a Phoenician prince famous in Greek mythology for, among other things, introducing writing and founding the city of Thebes. Why is a bitter victory called Cadmean? The story goes something like this: Cadmus, with his comrades, nears the site where he is to build Thebes, only to encounter a sacred dragon. The dragon kills all of Cadmus' pals, and he, coming upon the carnage, kills the dragon in turn. However, he has no one left to help him build the city, so sows the dragon's teeth to make more, only to find that they promptly set upon and kill each other. Only five remain, and these are the ancestors of the Thebans. Because of his slaying of the dragon, Zeus eventually turns Cadmus and his wife Harmonia into dragons. As is sometimes said of a Pyrrhic victory, "One more win like that and we're sunk."