ground attack units which are used against all types of targets in the battle area; bombs, rockets and cannon fire all being employed. It was usually the practice to use large numbers of aircraft to support a land attack in order to swamp the defences and to produce the effect of an artillery barrage. Fighters are used to protect the ground attack aircraft and to carry out normal offensive and defensive operations over the battlefield. Twin-engine bombers are also used mainly against communication targets and usually within about 50 miles of the front line.