During the preparation of the Normandy Invasion, the American high command realized soon enough that the 8th Air Force, which was wholly committed to strategic bombing, could not undertake the close support of ground units in this major offensive. The keystone of operation Overlord would indeed be mastery of the skies: tactical aviation had to be able to intervene quickly and precisely, and to operate from bases on the Continent. The heavy bombers, as they were dependent on large and permanent airfields, could not be called upon for such tasks. The concept of on-call tactical air support was...