This senior cabinet post formally emerged in the 20th century, first as minister of defence in 1936 and then secretary of state for defence in 1964.
Originally there were separate ministries for war, air and the Admiralty, each overseen by ministers below cabinet rank.
A minister with responsibility for defence was first appointed to the cabinet in 1936 but the position increased in importance in 1940 when Winston Churchill appointed himself Minister of Defence while also Prime Minister.
Churchill's successor as Prime Minister, Clement Attlee, also served as both Prime Minister and Minister of Defence.
The Ministry of Defence was established in 1946 when the title gained more formal recognition.
George Orwell, writing a year later in 1947, parodied the title of Attlee's new ministry in his novel Nineteen Eighty Four, calling it the Ministry of Peace - even though it was charged with maintaining a constant state of war.