Mithridates I, king of Parthia had taken advantage of the conflict between Demetrius and Tryphon to seize control of Susa and Elymais in 144 and of Mesopotamia in mid-141 BC. In 139/8, Demetrius journeyed east to reclaim these territories from the Parthians. He was initially successful, but was defeated in the Iranian mountains and taken prisoner in July or August of 138 BC. Parthian control of Mesopotamia was thus reaffirmed. In Syria, Tryphon was briefly left as uncontested ruler of the remaining Seleucid territories, but the Seleucid dynasty's grip was reestablished under Antiochus VII Sidetes, the younger brother of Demetrius, who also married Cleopatra Thea.