The principal administrative agency for the British army from 1683 until 1964, although it was not until 1855 that all administrative functions were centralized within it. After having several London homes, the War Office was located at the Horse Guards building in Whitehall from 1722 until 1858. It then moved to Pall Mall but it was only in 1906 that the office was moved to purpose-built accommodation in Whitehall. The War Office originated with the appointment of William Blathwayt as Secretary at War in 1683. Two predecessors had acted as clerks to the army's C-in-C but Blathwayt greatly extended his functions to include most of the routine day-to-day administration. However, the Secretary at War remained a minor official within government and, in war, strategic policy was directed by the principal secretaries of state presiding over the northern and southern departments. In 1782 the former became the Foreign Office and the latter the Home Office while, in 1801, overall responsibility for military and colonial affairs was vested in a new Secretary of State for War and Colonies. Responsibility for war and colonies was separated in 1854 and the new Secretary of State for War then absorbed the office of the Secretary at War in 1855.
Under the new arrangements, there was a duality of power between the Secretary of State for War and the army C-in-C but, as part of the Cardwell reforms, the latter was made subordinate to the Secretary for War and in 1871 the office of the C-in-C was symbolically moved from the Horse Guards to Pall Mall. In practice, the C-in-C retained considerable influence through the sheer permanence of the Duke of Cambridge, who held the office from 1855 until 1895, while war ministers were subject to the vagaries of the electoral system. A War Office Council was established in 1890 to widen the range of professional advice reaching the Secretary of State but the continuing defects in War Office organization apparent during the Second Boer War led to the recommendations by the Esher Committee in 1904 to abolish the C-in-C and appoint a chief of the general staff. A Committee of Imperial Defence (CID) had also emerged in 1902 as a forum for the discussion of wider defence issues. The advantages of the establishment of a general staff were lost during WW I when Kitchener, the only soldier ever to become Secretary of State, virtually dismantled it. After 1918 the CID became increasingly important and the appointment of a Minister for Co-ordination of Defence in 1936 and Churchill's decision to assume the post of Minister of Defence as well as that of PM in 1940 were indicative of the declining significance of the War Office. A single Ministry of Defence absorbing the War Office, Admiralty, and Air Ministry was then created in 1964.
— Ian Beckett
