The War of the Brothers was a civil war in the Seleucid Empire that followed the end of the Third Syrian War. That war had seen Seleucus II expel the Egyptians from the Syrian heartland of the empire, but while doing that he had had to appoint his half brother, Antiochus Hierax, as regent of the Seleucid lands in Asia Minor. At the end of the Third Syrian War Antiochus rebelled, in an attempt to seize control of the entire empire. He won support from some of the independent kingdoms of Asia Minor, amongst them Pontus and Cappadocia, although not from the increasingly important Pergamum.
Seleucus responded by invading Asia Minor. In the first year of campaigning he re-conquered Lydia, but was held at Sardes. The next year he moved against Mithridates II of Pontus. Antiochus responded by forming an alliance with the Gauls of Galatia, recently established in the bleak centre of Asia Minor. Strengthened by the Gauls, Antiochus inflicted a heavy defeat of Seleucus at Ancyra, in 240 or 239 BC. Seleucus escaped from the battlefield, but was eventually forced to acknowledge Antiochus as ruler of the Seleucid lands west of the Taurus. The war ended before 236 BC.
Antiochus then turned his attention to Pergamum, but his attempts to invade ended in defeat. He may have made another attempt to seize the Syrian heart of the Seleucid empire but failed, and was eventually murdered somewhere in Thrace in 226 BC. Seleucus was forced to spend the last ten years of his reign in the east of his empire, trying unsuccessfully to turn back the Parthian invasion. He too died in 226 BC. Pergamum retained control of Seleucid Asia Minor until it was regained by Antiochus III the Great.