Complex sentence: The German IX Reserve Corps had arrived from Belgium by 15 September and next day joined the 1st Army for an attack to the south-west with the IV Corps and the 4th and 7th cavalry divisions, against the attempted French envelopment.
Split into simpler sentences: The German IX Reserve Corps had arrived from Belgium by 15 September. Next day the corps joined the 1st Army, for an attack to the south-west with the IV Corps and the 4th and 7th cavalry divisions, against the attempted French envelopment.