This study in the Historical Analysis Series examines a subject of importance not only to the Army but also to the Air Force: the origin and development of American close air support doctrine and practice in World War II. Close air support doctrine both then and now is critical to the services. As this study demonstrates, the doctrine that had been conceived and practice prior to the first American battles of World War II fell apart in the mud and fog of Tunisia. Both air and ground commanders in 1941 recognized the necessity of close cooperation between the staffs and forces in joint and combined forces. What they had to learn in 1942 was the degree to which close air support doctrine tested that cooperation and required alteration. The struggle of ground and air leaders to define and construct a command and control system, and ultimately to allocate and commit precious air resources to requisite ground missions, has as many lessons today as it did more than forty years ago.