 Antiochus I (reigned 281–261 BC) and his son and successor Antiochus II Theos (reigned 261–246 BC) were faced with challenges in the west, including repeated wars with Ptolemy II and a Celtic invasion of Asia Minor—distracting attention from holding the eastern portions of the Empire together.Towards the end of Antiochus II's reign, various provinces simultaneously asserted their independence, such as Bactria and Sogdiana under Diodotus, Cappadocia under Ariarathes III, and Parthia under Andragoras.A few years later, the latter was defeated and killed by the invading Parni of Arsaces – the region would then become the core of the Parthian Empire.