The battle of Ancyra was the decisive battle of the War of the Brothers, a civil war within the Seleucid Empire that followed the Third Syrian War. Seleucus II had won control of the Syrian heartland of the Empire during that war, but had had to make his brother Antiochus Hierax regent in Asia Minor. At the end of the Syrian War Antiochus had then rebelled against Seleucus in an attempt to win control of the entire empire. He made alliances with Mithridates II of Pontus, Ariarathes III of Cappadocia and the Gauls of Galatia.
Seleucus responded by invading Asia Minor. After a successful campaign in either 241 or 240 BC, the next year he made a move against Mithridates. Antiochus responded with an army that contained a large Gallic contingent.
The two armies came together close to Ancyra (modern Ankara) in either 240 or 239 BC (depending on the date of Seleucus’s first campaign). Antiochus’s Gauls inflicted a heavy defeat of Seleucus, who was apparently lucky to escape back across the Taurus while his army was destroyed. At some point over the next two or three years the two brothers agreed to a peace settlement, which gave Antiochus control of the Seleucid lands in Asia Minor and Seleucus Syria and the eastern part of the empire.