Airborne forces are military units, usually light infantry, set up to be moved by aircraft and 'dropped' into battle. Thus they can be placed behind enemy lines, and have an ability to deploy almost anywhere with little warning. The formations are limited only by the number and size of their aircraft, so given enough capacity a huge force can appear "out of nowhere" in minutes, an action referred to as vertical envelopment.
Conversely, airborne forces typically lack the supplies and equipment for prolonged combat operations, and are therefore more suited for airhead operations than long-term occupation; furthermore, parachute operations are particularly sensitive to adverse weather conditions. Advances in helicopter technology since World War II have brought increased flexibility to the scope of airborne operations, and air assaults have largely replaced large-scale parachute operations, and (almost) completely replaced combat glider operations. However, due to the limited range of helicopters and the limited number of troops that can be transported by them many countries retain Paratroopers as a valuable strategic asset.