Air supremacy is the complete dominance of the air power of one side's air forces over the other side's, during a military campaign. It is the most favorable state of control of the air. It is defined by NATO and the United States Department of Defense as "that degree of air superiority wherein the opposing air force is incapable of effective interference." There are normally thought to be three levels of control of the air. Air supremacy is the highest, meaning there is complete control of the skies. Air superiority is the next highest, which is being in a more favorable position than the opponent. It is defined in the NATO Glossary as "That degree of dominance in the air battle of one force over another that permits the conduct of operations by the former and its related land, sea, and air forces at a given time and place without prohibitive interference by opposing air forces." Air parity is the lowest level of control, meaning control of the skies only above friendly troop positions. Air power has since become an increasingly powerful element of military campaigns; military planners view having at least an environment of air superiority as a necessity.