Complex sentence: The depleted 21st Brigade was withdrawn to Ioribaiwa Ridge where it was reinforced by the 3rd Battalion awaiting relief by the 25th Brigade, under command of Brigadier Kenneth Eather.
Split into simpler sentences: The depleted 21st Brigade was withdrawn to Ioribaiwa Ridge. It was reinforced by the 3rd Battalion and awaited relief by the 25th Brigade, under command of Brigadier Kenneth Eather.