Airpower or air power consists of the application of military aviation, military strategy and strategic theory to the realm of aerial warfare and close air support. Airpower began in the advent of powered flight early in the 20th century. Airpower represents a "complex operating environment that has been subjected to considerable debate". British doctrine defines airpower as "the ability to project power from the air and space to influence the behaviour of people or the course of events." The Australian Experience of Air Power defines Airpower as being composed of Control of the Air, Strike, Intelligence Surveillance and Reconnaissance, and Air Mobility roles.