Complex sentence: It has been a tradition for all researchers writing the history of the city to claim that the city dates back to the 3rd century BC and that it was founded by the Seleucid Dynasty, one of the Hellenistic kingdoms.
Split into simpler sentences: It has been a tradition for all researchers writing the history of the city to claim that the city dates back to the 3rd Cent. B.C. and that it was founded by the Seleucid Dynasty, one of the Hellenistic kingdoms.