A “Cadmean victory” is a victory in which the victor has suffered such great loss as hardly to be distinguished from the vanquished.
The term derives from the mythological founding of Thebes.
Cadmus, prince of Phenicia, searching for his lost sister, was advised by an oracle to abandon the search and to follow a cow and establish a city where she should lie down.
The cow led him into Bceotia where, before Cadmus could carry out the charge to found a city, his companions were all devoured by a dragon.
Cadmus thereupon slew the dragon and, at the command of the goddess Athene, scattered its teeth over the field.
Immediately armed men sprang from the teeth and fiercely turned upon the hero. To divert the threat, however, Cadmus induced them to turn upon each other.
When all but five were thus slain, Cadmus stopped the strange duel and persuaded the five to assist him in the founding of the city.
Hence also the expression, to sow dragon’s teeth, meaning, to sow seeds of strife and discord.