When the United States went to war in April 1917 the Army's Air Service had one squadron of obsolete aircraft. By November 1918 the Air Service had aero squadrons which were specialized in air combat, observation, bombing, and photography. Each combat division habitually had an air observation squadron and a balloon company attached. This work also details the efforts of the Air Service to construct a massive system of supply, repair, and maintenance. Questions such as the training of flyers, observers, and balloonists are also explored.