This thesis examines the origins and development of the Chiefs of Staff Sub-committee from its foundation in 1923 as a result of recommendations by the Salisbury Committee until the outbreak of World War II. Under the terms of their Warrant, the Chiefs of Staff were charged collectively to keep the defence situation under constant review and advise the Cabinet on strategic and military policy,. As 'Super Chiefs of a War Staff in Commission', they would act as the centre of strategic planning for the Committee of Imperial Defence,., Consulting 'together, they would co-ordinate plans and policy and evolve a common strategic doctrine The Chiefs of Staff Sub-committee did not realize these hopes or achieve the status of 'Super Chiefs of Staff in Commission'. The Ten Year Rule provided no political foundation for strategic policy and provoked such intense rivalry among the Chiefs of Staff, with each struggling to secure for his Service a larger share of the Treasury's diminished allowance, that long term strategic planning was never begun. Through their Annual Reviews, the Chiefs of Staff were instrumental in bringing about the abolition of the Ten Year Rule end the establishment of the Defence Requirements Committee to p=spare a programme to meet the Services' worst deficiencies. But the recommendations of the Chiefs of Staff were not based on any serious strategiC thought and feiled to make adequate provision for obvious crintingencies. As B result, mini3ters imposed radical changes on the programmes of the Defence Requirements Committee. As ministerial confidence in the abilities of the Chiefs of Staff end the quality of their professional advice declined, they exercised correspo~dingly less influence over defence: policy ond their strategic appreCiations, often prescient. were frequently· ignored,., The Abyssinian crisis demonstrated that the Chiefs of Staff and Joint Planners did not set the standard of a .combined General Staff~ From 1935 onwards, the unresolved controversy over the role of·air forces precluded a large measure of strategical and operational planning,.,The appointment of a Mini~ter for Co-ordination of Defence did little: to improve the organization of defe~ce or bring the .Chiefs of·Staff closer together o~ matters of principle~ That in war the Chiefs of staff would assume the role of 'Super Commanders in Commission' was expected; however, no Service Chief of the period had the training or experience demanded of the Supreme Commander, nor were these. qualities developed within the Chiefs of Staff Sub-committee.