Cadmean victory (n.)

c. 1600, "victory involving one's own ruin," translating Greek Kadmeia nikē, from Cadmus (Greek Kadmos), legendary hero-founder of Thebes in Boeotia and bringer of the original sixteen-letter alphabet to Greece. Probably a reference to the story of Cadmus and the "Sown-Men," who fought each other till only a handful were left alive. Compare Pyrrhic (adj.1).