Complex sentence: Meanwhile Parthia was invaded by the Scythians (the Tochari of Bactria), who had helped Antiochus VII. Phraates II marched against them, but was defeated and killed.
Split into simpler sentences: Meanwhile Parthia was invaded by the Scythians (the Tochari of Bactria), who had helped Antiochus. Phraates marched against them, but was defeated and killed.