The Army and the Air Force have long disputed the requirements and the control of the tactical air mission, especially close air support. Because of fundamental differences over the nature of close air support and higher USAF priorities, the Air Force has not always succeeded in meeting Army requirements for close air support. Because it views air operations as an integrated whole, the Air Force has defined close air support in terms of the overall tactical air mission, while the Army has defined it in terms of the mission of the ground forces engaged in battle. Both services consider close air support an essential part of their missions and capabilities — the Army now has its own force of about 12,000 aircraft. Thus, an important issue is whether these capabilities should be duplicative or complementary. The most likely prospect seems to be the evolution of a modus vivendi for sharing the function between the services.