A Cadmean victory (Greek: Kadmeia nike) is a reference to a victory involving one's own ruin, from Cadmus (Greek: Kadmos), the legendary founder of Thebes in Boeotia and the mythic bringer of script to Greece. On seeking to establish the city, Cadmus required water from a spring guarded by a water-dragon similar to the Lernaean Hydra. He sent his companions to slay the dragon, but they all perished. Although Cadmus eventually proved victorious, the victory cost the lives of those who were to benefit from the new settlement.
The phrase "Cadmean victory" has been largely displaced in popular use by the somewhat similar expression "to win a battle but lose the war".
References
Cadmean victory Wikipedia(Text) CC BY-SA