Reconnaissance Assault Vehicles; by 1970 over 200 Sheridan tanks were operating in Vietnam. 21st century Tanks rarely work alone; the usual minimum unit size is a platoon (a platoon is the smallest U.S. Army/Marine unit led by an officer, and a component of a company or troop) of three to five tanks. The tanks of the platoon work together providing mutual support: two might advance while covered by the others then stop and provide cover for the remainder to move ahead. Normally, multiple platoons coordinate with mechanised infantry and use their mobility and firepower to penetrate weak points in enemy lines. This is where the powerful engines, tracks and turrets come into play. The ability to rotate the turret by a full 360° allows coordinated movement within and between platoons, while defending against attacks from multiple directions and engaging troops and vehicles without stopping or slowing down. When on the defensive, they wait in prepared positions or use any natural terrain elements (such as small hills) for cover. A tank sitting just behind a hill crest ("hull-down") exposes only the top of its turret, with the gun and sensors, to the enemy, leaving the smallest possible target while allowing it to engage the enemy on the other side of the hill. Tanks are usually able to depress the main gun below the horizontal since modern kinetic energy (KE) rounds have nearly flat trajectories. Without this they would be unable to exploit such positions. However, upon cresting a hill, the tank may expose its thinly armoured underside to enemy weapons. The deposition of armour around a tank is not uniform; the front is typically better armoured than the sides or rear. Accordingly, normal practice is to keep the front towards the enemy at all times; the tank retreats by reversing instead of turning around. Driving backwards away from an enemy is even safer than driving forwards towards them since driving forwards over a bump can throw the front of the tank up in the air, exposing the thin armour of the underside and taking the gun off the target due to its limited angle of depression. The tracks, wheels and suspension of a tank are outside the armoured hull and are some of the most vulnerable spots. The easiest way to disable a tank (other than a direct hit in a vulnerable area with a full-power anti-tank weapon) is to target