Airpower or air power comprises the application of military strategy and strategic theory to the realm of aerial warfare. Airpower is difficult to define and lacks a widely agreed definition as it 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." By any definition, airpower reasonably only began with the advent of mechanized flight (early 20th century).
Airpower is a function of air supremacy and numbers. Roughly speaking, a combatant side that has 100% or near 100% control of the skies has air supremacy; an advantage of some 70-90% would indicate air superiority. A 50/50 split is air parity; lower than this, one side may be said to be air denied or air incapable. Because airplanes generally take off from designed airfields on missions typically involving some hours of cruising, the precise state of air superiority is fluid and less defined vis-a-vis land or sea combat. For example, a contested airspace directly above a battlezone bristling with antiaircraft weapons may be denied to the airforces of both sides. Further, the completely different situations of a technologically advanced airforce with one flight of high-tech planes (air supremacy but low capability) or a low-tech force of massive numbers of low-tech planes (e.g. An-2) resulting in high capability but low long-term survivability demonstrate that 'air power' is multi-faceted and complex.
Significant contributors are Giulio Douhet, Billy Mitchell, John Boyd and John A. Warden III.
Some early strategists/visionaries after World War I imagined that airpower alone would suffice to bring nations to their knees. The Bombing of Guernica was an early trial that revealed both capabilities and limitations. But yet another maxim 'no war was ever won solely by airpower' was challenged by the NATO victory in Kosovo. Airpower has been used to conduct lightning strategic strikes, to complement land offensives, to instill fear/lower morale similarly to a fleet in reserve, and to create broad-based destruction behind enemy lines. With airpower, supplies can be transported by cargo planes, providing a decisive edge in mobility. Military and civilian aircraft, moreover, interact in a number of complex ways, including shootdowns of civilian planes whether mistaken or not; military escorts of civilian planes; civilian planes being used for military transport, espionage, or other purposes; and/or no-fly zones being enforced to punish or sanction a target nation. Airpower also relates to space power, although militarization of space remains regulated by international treaty.
Western developed nations have enjoyed a consistent advantage in airpower since the beginning of mechanized flight. Airpower has been wielded mostly decisively in the last hundred years by Germany, the United Kingdom, the United States, the Soviet Union, Japan, Italy, and France, with many client nations using aircraft developed by one or more of these nations. A mass technological base is considered necessary for the development of airpower.