Following division of Alexander's empire, Seleucus I Nicator received Babylonia. From there, he created a new empire which expanded to include much of Alexander's near eastern territories. At the height of its power, it included central Anatolia, the Levant, Mesopotamia, Persia, today's Turkmenistan, Pamir, and parts of Pakistan. It included a diverse population estimated at fifty to sixty million people. Under Antiochus I (c. 324/3 – 261 BC), however, the unwieldy empire was already beginning to shed territories. Pergamum broke away under Eumenes I who defeated a Seleucid army sent against him. The kingdoms of Cappadocia, Bithynia and Pontus were all practically independent by this time as well. Like the Ptolemies, Antiochus I established a dynastic religious cult, deifying his father Seleucus I. Seleucus, officially said to be descended from Apollo, had his own priests and monthly sacrifices. The erosion of the empire continued under Seleucus II, who was forced to fight a civil war (239-236) against his brother Antiochus Hierax and was unable to keep Bactria, Sogdiana and Parthia from breaking away. Hierax carved off most of Seleucid Anatolia for himself, but was defeated, along with his Galatian allies, by Attalus I of Pergamon who now also claimed kingship.
What was the name of Seleucus II brother, whom he fought in civil war?
Antiochus Hierax