Complex sentence: Sapping is a term used in siege operations to describe any trench excavated under defensive musket or artillery fire that was intended to advance a besieging army's position in relation to the works of an attacked fortification.
Split into simpler sentences: Sapping is a term used in siege operations to describe any trench excavated near an attacked, defended fortification, under defensive small arms or artillery fire. The trench, referred to as a "sap", is intended to advance a besieging army's position in relation to the works of an attacked fortification.