239 BC[1] between the Seleucid King Seleucus II Callinicus and his brother Prince Antiochus Hierax.
[2] Civil war had raged in the Seleucid Empire since 244 BC, when Queen Laodice I had supported her son Antiochus in a rebellion against Seleucus.
He was able to take advantage of his brother's distraction and defeat at the hands of Ptolemy III in the Third Syrian War.
After the war had lasted two years or so, during which we have no information on its progress, Seleucus was defeated in a great battle against the united forces of Mithridates II of Pontus, Hierax and the Galatians near Ancyra, in around 239 BC.
[3] Antiochus' victory at Ancyra (modern Ankara, Turkey) forced Seleucus to relinquish his territories in Anatolia to him and content himself with the Seleucid lands east of the Taurus mountains.