From the Document: "Almost since the invention of heavier-than-air flight, control of the air has been seen as a military advantage. Over time, the United States has come to treat air superiority as a necessity, and built such capable air forces that no enemy aircraft has killed U.S. ground troops since 1953. Modern airpower is able to provide a full range of effects, from strategic operations at intercontinental ranges to direct support of troops in combat. Today, every branch of the U.S. military employs air forces for various purposes, employing nearly 14,000 aircraft."
CRS In Focus, IF10546
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html