Complex sentence: The cemetery was begun on 4th July 1916, employing a section of the old front line trench in Mansel Copse that had been held by troops of the 9th Devons.
Split into simpler sentences: The cemetery was begun on 4th July 1916, employing a section of the old front line trench in Mansel Copse. This trench had been held by troops of the 9th Devons.