On 29 September 1939, soon after the start of the Second World War, Rear Admiral John Godfrey, the Director of Naval Intelligence, circulated the Trout memo, a paper that compared the deception of an enemy in wartime to fly fishing.

In August 1942, before the Battle of Alam el Halfa, a corpse was placed in a blown-up scout car, in a minefield facing the German 90th Light Division. On the corpse was a map purportedly showing the locations of British minefields; the Germans used the map, and their tanks were routed to areas of soft sand where they bogged down.

In September 1942 an aircraft flying from Britain to Gibraltar crashed off Cádiz. All aboard were killed, including Paymaster-Lieutenant James Hadden Turner – a courier carrying top secret documents – and a French agent.

In late 1942, with the Allied success in the North African campaign, military planners turned their attention to the next target. British planners considered that an invasion of France from Britain could not take place until 1944 and the Prime Minister, Winston Churchill, wanted to use the Allied forces from North Africa to attack Europe's "soft underbelly".

At the Casablanca Conference in January 1943, Allied planners agreed on the selection of Sicily – codenamed Operation Husky – and decided to undertake the invasion no later than July. There was concern among the Allied planners that Sicily was an obvious choice – Churchill is reputed to have said "Everyone but a bloody fool would know that it's Sicily"– and that the build-up of resources for the invasion would be detected.

On 28 January 1943 Bentley Purchase contacted Montagu with the news he had located a suitable body, probably that of Glyndwr Michael, a tramp who died from eating rat poison that contained phosphorus.

Montagu selected the code name Mincemeat from a list of centrally held available possibilities. On 4 February 1943 Montagu and Cholmondeley filed their plan for the operation with the Twenty Committee; it was a re-working of Cholmondeley's Trojan Horse plan.

On 13 April 1943 the committee of the Chiefs of Staff met and agreed that they thought the plan should proceed. The committee informed Colonel John Bevan – the head of London Controlling Section, which controlled the planning and co-ordination of deception operations – that he needed to obtain final approval from Churchill.

In the early hours of 17 April 1943 the corpse of Michael was dressed as Martin, although there was one last-minute hitch: the feet had . Purchase, Montagu and Cholmondeley could not put the boots on, so an electric heater was located and the feet defrosted enough to put the boots on properly. The pocket litter was placed on the body, and the briefcase attached.

Seraph set sail and arrived just off the coast of Huelva on 29 April after having been bombed twice en route. After spending the day reconnoitring the coastline, at 4:15 am on 30 April, Seraph surfaced. Jewell had the canister brought up on deck, then sent all his crew below except the officers.

The body of "Major Martin" was found at around 9:30 am on 30 April 1943 by a local fisherman; it was taken to Huelva by Spanish soldiers, where it was handed over to a naval judge. Haselden, as vice-consul, was officially informed by the Spaniards; he reported back to the Admiralty that the body and briefcase had been found.

On 30 April 1943, Lt. Norman Jewell, captain of the submarine HMS Seraph, read the 39th Psalm, and Martin's body was gently pushed into the sea where the tide, aided by the push of the submarine's propellers, would bring it ashore off Huelva on the Spanish Atlantic coast.

At midday on 1 May an autopsy was undertaken on Michael's body; Haselden was present and – in order to minimise the possibilities that the two Spanish doctors identified that the body was a three-month-old corpse – Haselden asked if, in the heat of the day and smell of the corpse, the doctors should bring the post mortem to a close and have lunch.

On 5 May, the briefcase was passed to the naval headquarters at San Fernando near Cadiz, for forwarding to Madrid.

On 11 May the briefcase, complete with the documents, was returned to Haselden by the Spanish authorities; he forwarded it to London in the diplomatic bag.

On 14 May 1943 Grand Admiral Karl Dönitz met Hitler to discuss Dönitz's recent visit to Italy, his meeting with the Italian leader Benito Mussolini and the progress of the war.

On 9 July the Allies invaded Sicily in Operation Husky. German signals intercepted by GC&CS showed that even four hours after the invasion of Sicily began, twenty-one aircraft left Sicily to reinforce Sardinia. For a considerable time after the initial invasion, Hitler was still convinced that an attack on the Balkans was imminent, and in late July he sent General Erwin Rommel to Salonika to prepare the defence of the region. By the time the German high command realised the mistake, it was too late to make a difference.

On 25 July 1943, as the battle for Sicily went against the Axis forces, the Italian Grand Council of Fascism voted to limit the power of Mussolini, and handed control of the Italian armed forces over to King Victor Emmanuel III. The following day Mussolini met the King, who dismissed him as prime minister; the former dictator was then imprisoned. A new Italian government took power and began secret negotiations with the Allies.

Montagu was appointed an Officer of the Order of the British Empire in 1944 for his part in Operation Mincemeat; for masterminding the plan, Cholmondeley was appointed a Member of the Order in 1948.

Operation Mincemeat is a 2021 British war drama film directed by John Madden. It is based upon Ben Macintyre's book on the British Operation Mincemeat during the Second World War. The film stars Colin Firth, Kelly Macdonald, Matthew Macfadyen, Penelope Wilton, Johnny Flynn, and Jason Isaacs. This marks Paul Ritter’s final film appearance following his death and was dedicated to his memory. Production companies were Film Nation Entertainment and Cross City Films and See-Saw Films and Cohen Media Group and Archery Films.