Paratroopers are military parachutists — military personnel trained in parachuting into an operation and usually functioning as part of an airborne force. Military parachutists (troops) and parachutes were first used on a large scale during World War II for troop distribution and transportation. Paratroopers are often used to seize strategic objectives such as airfields or bridges. They are used for tactical advantage as they can be inserted into the battlefield from the air, thereby allowing them to be positioned in areas not accessible by land. They jump out of airplanes using their parachutes to land safely on the ground. It is one of the three types of "forced entry" strategic techniques for entering a theater of war; the other two are by land and sea. The ability of air assault to enter the battlefield from any location allows paratroopers to evade emplaced fortifications that exist to prevent an attack from a specific direction, and the possible use of paratroopers, forces an army to spread their defenses to protect other areas which would otherwise be safe by geographical virtue. Another common use for paratroopers is to establish an airhead for landing other units. This doctrine was first practically applied to warfare by the Italians and the Soviets. During World War II, however, the two countries' ground forces were often overstretched, leaving their elite paratroopers to be employed as regular infantry. The first extensive use of paratroopers (Fallschirmjäger) was by the Germans during World War II and later in the war also by the Americans and the British Commonwealth. Owing to the limited capacity of cargo aircraft of the period (for example the Ju-52) they rarely, if ever, jumped in groups much larger than 20 from one aircraft. In English language parlance, this load of paratroopers is called a "stick", while any load of soldiers gathered for air movement is known as a "chalk". The terms come from the common use of white chalk on the sides of aircraft and vehicles to mark and update numbers of personnel and equipment being emplaned. In World War II, paratroopers most often used parachutes of a circular or round design. These parachutes could be steered to a small degree by pulling on the risers (four straps connecting the paratrooper's harness to the connectors) and suspension lines which attach to the parachute canopy itself. German paratroopers, whose harnesses had only a single riser attached at the back, could not manipulate their parachutes in such a manner. Today, paratroopers still use round parachutes, or round parachutes modified as to be more fully controlled with toggles. The parachutes are usually deployed by a static line. Mobility of the parachutes is often deliberately limited to prevent scattering of the troops when a large number parachute together. Some military exhibition units and special forces units use "ram-air" parachutes, which offer a high degree of maneuverability and are deployed manually (without a static line) from the desired altitude.