Originally housed in the Crystal Palace at Sydenham Hill, the museum opened to the public in 1920. In 1924, the museum moved to space in the Imperial Institute in South Kensington, and finally in 1936, the museum acquired a permanent home that was previously the Bethlem Royal Hospital in Southwark. The outbreak of the Second World War saw the museum expand both its collections and its terms of reference, but in the post-war period, the museum entered a period of decline. The 1960s saw the museum redevelop its Southwark building, now referred to as Imperial War Museum London, which serves as the organisation's corporate headquarters. During the 1970s, the museum began to expand onto other sites. The first, in 1976, was a historic airfield in Cambridgeshire now referred to as IWM Duxford. In 1978, the Royal Navy cruiser HMS Belfast became a branch of the museum, having previously been preserved for the nation by a private trust. In 1984, the Cabinet War Rooms, an underground wartime command centre, was opened to the public. From the 1980s onwards, the museum's Bethlem building underwent a series of multimillion-pound redevelopments, completed in 2000. Finally, 2002 saw the opening of IWM North in Trafford, Greater Manchester, the fifth branch of the museum and the first in the north of England. In 2011, the museum rebranded itself as IWM, standing for "Imperial War Museums".
The museum's collections include archives of personal and official documents, photographs, film and video material, and oral history recordings, an extensive library, a large art collection, and examples of military vehicles and aircraft, equipment, and other artefacts.
The museum is funded by government grants, charitable donations, and revenue generation through commercial activity such as retailing, licensing, and publishing. General admission is free to IWM London (although specific exhibitions require the purchase of a ticket) and IWM North, but an admission fee is levied at the other branches. The museum is an exempt charity under the Charities Act 1993 and a non-departmental public body under the Department for Culture, Media and Sport. As of January 2012, the Chairman of the Trustees is Sir Francis Richards. Since October 2008, the museum's Director General has been Diane Lees.
On 27 February 1917 Sir Alfred Mond, a Liberal MP and First Commissioner of Works, wrote to the Prime Minister David Lloyd George to propose the establishment of a National War Museum. This proposal was accepted by the War Cabinet on 5 March 1917 and the decision announced in The Times on 26 March. A committee was established, chaired by Mond, to oversee the collection of material to be exhibited in the new museum.
This National War Museum Committee set about collecting material to illustrate Britain's war effort by dividing into subcommittees examining such subjects as the Army, the Navy, the production of munitions, and women's war work. There was an early appreciation of the need for exhibits to reflect personal experience in order to prevent the collections becoming dead relics. Sir Martin Conway, the Museum's first Director General, said that exhibits must 'be vitalised by contributions expressive of the action, the experiences, the valour and the endurance of individuals'. The museum's first curator and secretary was Charles ffoulkes, who had previously been curator of the Royal Armouries at the Tower of London. In July 1917 Mond made a visit to the Western Front in order to study how best to organise the museum's growing collection. While in France he met French government ministers, and Field Marshal Haig, who reportedly took great interest in his work. In December 1917 the name was changed to the Imperial War Museum after a resolution from the India and Dominions Committee of the museum.