A short while later the Parni seized the rest of Parthia from Andragoras, killing him in the process. With the conquest of the province, the Arsacids became known as Parthians in Greek and Roman sources. This term has likewise been in regular use by modern western authors as well, however, according to the modern historian Stefan R. Hauser, it "should be abandoned as it conveys an incorrect idea of an ethnic ruling class within the multiethnic, multilingual population". The neighbouring province of Hyrcania was shortly conquered by the Parni as well. A recovery expedition by the Seleucids under Seleucus II was made in 228 BC, which proved problematic for Arsaces, who was at the same time at war with the Greco-Bactrian ruler Diodotus II (). In order to avoid fighting on two fronts, Arsaces quickly concluded a peace treaty with Diodotus II.