Complex sentence: Cappadocia under Ariarathes IV (220--163 BC) was initially allied with the Seleucids in their war against Rome, however Ariarathes changed alliances following the Battle of Magnesia (190 BC) becoming Rome's friend and joined Pergamon against Pontus.
Split into simpler sentences: Cappadocia under Ariarathes IV (220--163 BC) was initially allied with the Seleucids in their war against Rome. However, Ariarathes changed alliances following the Battle of Magnesia (190 BC), becoming Rome's friend, and joined Pergamon against Pontus.