Arising from the armistice concluded by France with Germany in June 1940 and the evacuation of British forces from the Continent, a small organisation was established to take command of subsequent raiding operations against enemy territory and to provide advice on combined assaults. From this emerged a distinct Combined Operations Headquarters, staffed by all three services, but independent of all of them and under the command of a Director of Combined Operations. Admiral of the Fleet, Sir Roger Keyes, was appointed first director in July 1940; he was succeeded in October 1941 by Commodore Lord Louis Mountbatten with the title Adviser on Combined Operations. In March 1942 this title was altered to Chief of Combined Operations; it was also decided that the Chief of Combined Operations should attend meetings of the Chiefs of Staff as a full member whenever major issues were in question and, as previously, when his own combined operations or any special matters in which he was concerned were under consideration.
From the establishment of a Combined Training Centre in August 1940 at Inveraray, Argyllshire, the Combined Operations Command expanded rapidly both within the United Kingdom and overseas, notably in the Middle East and India. In 1942 it sent a permanent representative to the Joint Staffs Mission in Washington and in the same year a Combined Operations Experimental Establishment was set up at Appledore, Devon. This establishment was much involved in the investigation of problems likely to be encountered on the beaches in connection with an invasion of Europe, particularly as regards the landing of armoured vehicles, stores, supplies, etc. Following the successful invasion of Normandy in 1944, a similar establishment was set up in India to carry out the developments and trials necessary in the very different conditions in the Far East.
From its formation, Combined Operations Headquarters maintained a close, though sometimes strained, contact on the naval side with the Admiralty, which set up a number of combined operations branches within its own departments, particularly in relation to operations, materials and personnel. Raiding forces, such as commandos, came under the command of Combined Operations Headquarters, except when they were employed as part of larger operations. Throughout the war Combined Operations Headquarters played a key role in the development of offensive operations against the enemy. This was notably the case in the raid on Dieppe in August 1942 and the preparation and planning of the North Africa and Sicily campaigns in 1942 to 1943, the invasion of Europe in 1944 and similarly, through its directorate in India, in operations in the Far East.
Following the war it was the Admiralty view that Combined Operations Headquarters should cease to be an independent organisation and should be replaced by a joint Combined Operational Planning Staff within the Chiefs of Staff organisation. In 1947, however, it was decided that Combined Operations Headquarters should continue to be responsible for policy, training and technique in amphibious warfare under the direction of the Chiefs of Staff; at the same time the title of chief of combined operations was changed to Chief of Combined Operations Staff and responsibility for Combined Operations estimates was transferred from the Service ministries to the newly-established Ministry of Defence. On 1 April 1948 Combined Operations Headquarters was placed under the administration of that ministry and in 1951 it was re-named Amphibious Warfare Headquarters.