The seminal military theorist Clausewitz wrote before the evolution of air power and did not pay much attention to the dynamics of the kinds of economic and maritime war traditionally waged by the British state in the eighteenth and early nineteenth centuries that were echoed in the maritime blockade and peripheral campaigns of World War II. Nonetheless, some of his ideas can be successfully applied to the British experience in that conflict. This narrative has used a series of case studies to provide a more general reappraisal of the Anglo-American aircraft supply relationship in the first half of World War II. All these case studies have identified factors operating to constrain the value of American aircraft supply. These constraints operated in the classical sense of Clausewitzian ‘friction’, or the accumulation of difficulties and circumstances that cause disappointment in any endeavour.

The result of this friction was that American aircraft supply in 1940–2 fell short of the assertions associated with it at the time. Furthermore, subsequent historical accounts failed to recognise fully this friction and have perpetuated misperceptions concerning the importance of American aircraft supply to Britain.