 Complete the following phrase:  According to James Howard-Johnston, from the third century BC to the early seventh century AD, the rival players in the East were grand polities with imperial pretensions, which had been able to establish and secure stable territories transcending regional divides. The Romans and Parthians came into contact through their respective conquests of parts of the Seleucid Empire. During the 3rd century BC, the Parthians migrated from the Central Asian steppe into northern Iran. Although subdued for a time by the Seleucids, in the 2nd century BC they broke away, and established an independent state that steadily expanded at the expense of their former rulers, and through the course of the 3rd and early 1st century BC, they had conquered Persia, Mesopotamia, and Armenia. Ruled by the Arsacid dynasty, the Parthians fended off several Seleucid attempts to regain their lost territories, and established several eponymous branches in the Caucasus, namely the Arsacid dynasty of Armenia, the Arsacid dynasty of Iberia, and the Arsacid dynasty of Caucasian Albania. Meanwhile, the Romans expelled the Seleucids from their territories in Anatolia in the early 2nd century BC, after defeating Antiochus III the Great at Thermopylae and 