Complex sentence: He conducted the retreat from Quatre Bras on 17 June and marked out the position the troops were to occupy at Waterloo on the 18th--to the rear, apparently, of the ground originally chosen by the duke.
Split into simpler sentences: He conducted the retreat from Quatre Bras on 17 June and marked out the position the troops were to occupy at Waterloo on the 18th June. This was, apparently, of the ground originally chosen by the duke.