Parthian Beginnings

 Parthia was originally a part of the Seleucid empire, which was one of the successor realms to Alexander the Great's huge empire.  The Seleucid empire began to slowly disintigrate during the 3rd century BC, first with the seccession of Bactria under Diodotus I in the 250s and then Parthia under Arsaces I. The Seleucid empire remained the gravest threat to Parthian independence until the middle of the 2nd century BC, despite wars with Bactria and invading tribes from the north and east.