Complex sentence: This gives attackers an overwhelming advantage: defenders can be attacked from all sides with artillery and/or machinegun fire, but they cannot easily be re-supplied (with food, ammunition and medical supplies etc) or escape.
Split into simpler sentences: This gives attackers an overwhelming advantage: defenders can be attacked from all sides with artillery and/or machinegun fire. However, since the defenders are completely surrounded, they cannot easily be re-supplied (with food, ammunition and medical supplies etc) or escape.