Probably referring to the battle in which the soldiers who sprang from the dragon's teeth sown by Cadmus slew each other.
Cadmean victory (plural Cadmean victories)
Part or all of this entry has been imported from the 1913 edition of Webster’s Dictionary, which is now free of copyright and hence in the public domain. The imported definitions may be significantly out of date, and any more recent senses may be completely missing.
(See the entry for “Cadmean victory”, in Webster’s Revised Unabridged Dictionary, Springfield, Mass.: G. & C. Merriam, 1913, →OCLC.)