What is the meaning of "what" in this text?
Ctesiphon, indeed, was not to Persia what it had been to Parthia; but still it might fairly be looked upon as a prize of considerable importance. Of Parthia it had been the main, in later times perhaps the sole, capital; to Persia it was a secondary rather than a primary city, the ordinary residence of the court being Istakr, or Persepolis.
From The Seven Great Monarchies of the Ancient World, vol. 7, chapter 10, by George Rawlinson.