This narrative will deal with tactical phases of the history of the corps observation branch of the American Air Service operating on the western front [of World War I]. Particular stress will be laid upon the development of the Corps Air Service in its intimate relation to other branches and arms of the service. This history will tell, campaign by campaign, in sequence: (a) What the Corps Air Service was; its organization. (b) What its mission was under varying conditions. (c) How it planned to accomplish its mission. (d) The operations undertaken in accordance with the plan. (e) The work actually accomplished. (f) Comments and criticisms, campaign by campaign. A final commentary in the form of a digest will conclude the history.