The Seleucid Empire

 The Seleucid Empire was founded by Seleukos I, one of Alexander the Great's commanders, in 312 BC.  In the aftermath of Alexander's death in 323 BC, Seleucos was given the satrapy of Babylon to rule in the name of Alexander's brother and son.  During the period of the wars of the Diadochi (the successors of Alexander), Seleukos managed to survive and to eventually establish the largest of the three lasting successor states, the Seleucid empire, which originally encompassed the whole of the eastern portions of Alexander's empire plus Syria and Asia Minor.  This was the empire from which Parthia gained its independence, and from which it inherited its bureacracy and Greek institutions.  The Seleucid threat to Parthian independence was finally eliminated by Mithradates I.