This thesis posits that although national defence strategy must ultimately be determined by civilian ministers its roots lie in the conceptual foundations of the professional armed services, which represent the primary sources of strategic advice. Consequently, the relative depth and coherence of these foundations are important factors in shaping options presented for political decision. These foundations comprise not only the body of doctrine that articulates the ways in which armed force can be applied most effectively to achieve specified ends, but also the institutions through which these ideas are assimilated within the services, and the staff structures that translate them into candidate strategies. To illustrate this process, this thesis evaluates the relative states of developments in the conceptual foundations of the Royal Navy and the British Army in the period from end of the war in South Africa in 1902 to the outbreak of the First World War in 1914. The first chapter examines how, having come to dominate defence policy on the back of external conceptions of naval power, the Navy was slow to develop its own capacity for conceptual development. The second chapter goes on to examine how its further refusal to create a strategic planning staff affected its ability to adapt to strategic change and to engage in vital debates over national strategy. The third chapter charts how the Army, despite its subordinate status and the ignominy of its failings in South Africa, was able to draw upon, and further develop, the capacity for conceptual development enshrined in the Staff College. The fourth chapter then examines how this foundation enabled the establishment of a credible strategic planning staff, which was instrumental to the structural reform of the service and the adaptation and communication of military strategy in the light of strategic reorientation. Ultimately, the thesis concludes that the conceptual disparity between the Navy and the Army has been neglected as a material factor in determining British strategy prior to the First World War.