The MOD states that its principal objectives are to defend the United Kingdom of Great Britain and Northern Ireland and its interests and to strengthen international peace and stability. With the collapse of the Soviet Union and the end of the Cold War, the MOD does not foresee any short-term conventional military threat; rather, it has identified weapons of mass destruction, international terrorism, and failed and failing states as the overriding threats to Britain's interests. The MOD also manages day-to-day running of the armed forces, contingency planning and defence procurement.
During the 1920s and 1930s, British civil servants and politicians, looking back at the performance of the state during World War I, concluded that there was a need for greater co-ordination between the three services that made up the armed forces of the United Kingdom—the Royal Navy, the British Army and the Royal Air Force. The formation of a united ministry of defence was rejected by David Lloyd George's coalition government in 1921; but the Chiefs of Staff Committee was formed in 1923, for the purposes of inter-service co-ordination. As rearmament became a concern during the 1930s, Stanley Baldwin created the position of Minister for Co-ordination of Defence. Lord Chatfield held the post until the fall of Neville Chamberlain's government in 1940; his success was limited by his lack of control over the existing Service departments and his limited political influence.
Winston Churchill, on forming his government in 1940, created the office of Minister of Defence to exercise ministerial control over the Chiefs of Staff Committee and to co-ordinate defence matters. The post was held by the Prime Minister of the day until Clement Attlee's government introduced the Ministry of Defence Act of 1946. The new ministry was headed by a Minister of Defence who possessed a seat in the Cabinet. The three existing service Ministers—the First Lord of the Admiralty, the Secretary of State for War and the Secretary of State for Air—remained in direct operational control of their respective services, but ceased to attend Cabinet.
From 1946 to 1964 five Departments of State did the work of the modern Ministry of Defence: the Admiralty, the War Office, the Air Ministry, the Ministry of Aviation, and an earlier form of the Ministry of Defence. These departments merged in 1964; the defence functions of the Ministry of Aviation Supply merged into the Ministry of Defence in 1971.