In 1936, Slessor published "Air Power and Armies", an examination of the use of air power against targets on and behind the battlefield. In this work he advocated army co-operation, interdiction to cut off enemy reinforcements and supply, and the use of aerial bombardment as a weapon against enemy morale. He did, however, acknowledge the limitations of his theory, stating:...the conditions envisaged throughout book are those of a campaign on the land in which the primary problem at the time is the defeat of an enemy army in the field. ... in a war against a great Naval power at sea, or when the principle threat to the Empire at the time is the action of hostile air forces against this country or its possessions, the aim and objectives of the air forces of the Empire will not be the same as described in this book. On 17 May 1937, following his posting to India, Slessor was promoted acting group captain, and appointed Deputy Director of Plans at the Air Ministry. He was promoted to substantive group captain on 1 July 1937. Mentioned in despatches on 18 February 1938, he took over as Director of Plans on 22 December 1938. He was appointed Air Aide-de-Camp to the King on 1 January 1939.