'Peppermint' was the US preparatory but in the event unrequired undertaking of the 'Manhattan Project' and the European Theater of Operations US Army to counter the danger that the Germans might disrupt the 'Neptune' (iii) and 'Overlord' Allied landings of June 1944 in Normandy with radioactive poisons (April 1944/May 1945).

In response this this possible threat, the Metallurgical Laboratory in Chicago and the Victoreen Instrument Company in Cleveland developed portable radiation-detection devices suitable for use in the field. In 1944, Major General Leslie R. Groves, director of the 'Manhattan Project', sent Major Arthur V. Peterson to brief General Dwight D. Eisenhower and his senior staff officers at the Supreme Headquarters Allied Expeditionary Force.

When the 'Manhattan Project' assumed responsibility for the development of nuclear weapons in September 1942, it also assumed responsibility for the development of suitable countermeasures as, at this time the threat posed by the German nuclear energy project was taken very seriously. Consideration was given to issuing a public warning of the danger of a German nuclear attack on the USA, but the director of the 'Manhattan Project', Brigadier General Leslie R. Groves, considered the likelihood of such a situation to be sufficiently remote that he rejected the notion of taking so drastic a step.

A subcommittee of the S-1 Executive Committee, chaired by James B. Conant, and comprising Conant himself, Arthur Compton and Harold Urey, was appointed to look into the issue, and it similarly assessed the danger as low but still sufficient to warrant taking some precautions. A programme was launched by the Metallurgical Laboratory in Chicago and the Victoreen Instrument Company in Cleveland to develop radiation detection devices suitable for use in the field. Some 48 portable detection meters were manufactured in 1943, half of which were capable of detecting 0 to 10 roentgens per day, while the other half could detect from 0 to 100 roentgens per day. The instrument sets were stored at Manhattan District offices in Boston, Chicago, New York City, San Francisco and Washington, DC, and the area engineer and some other officers were instructed in their use. A special team of scientists was created at the Metallurgical Laboratory to respond to any reports of the use of nuclear weapons or radioactive poisons.

It was considered more likely that Germany might employ such weapons against the UK, so four ETOUSA officers, including the head of the formation’s operations department, Brigadier General George S. Eyster, were called to Chicago for a top-secret briefing by the Manhattan District’s Chicago area engineer, Major Arthur V. Peterson. They were told about possible forms such an attack might take, informed of such an attack’s effects and symptoms, and given survey instruments together with instruction of their use. They were enjoined to tell other officers in the theatre to report unexplained fogging of film or illnesses with symptoms corresponding to the effects of radiation sickness.

Early in 1944, as the date for 'Overlord' campaign in Normandy approached, Groves considered the risk sufficient for the despatch of an officer to brief the Allied supreme commander, General Dwight D. Eisenhower, of the possible use of radioactive poisons, particularly plutonium and fission products that might be created in German nuclear reactors. On 8 April, Peterson reported to the Supreme Headquarters Allied Expeditionary Force to meet Eisenhower, his chief-of-staff, Lieutenant General Walter Bedell Smith, the assistant chief-of-staff (intelligence), Major General John Whiteley, and the assistant chief-of-staff (operations), Major General Harold R. Bull.

These men considered the creation of an Allied plan to counter the danger, but Whiteley said that he would have to consult with the British command before this could be approved. Eyster was then ordered to prepare a US plan as 'Peppermint'. Further briefings were given to Admiral Harold Stark, the commander of the US Naval Forces in Europe, and Lieutenant Generals Carl A. Spaatz and John C. H. Lee, and Eisenhower also wrote to Lieutenant General Sir Hastings Ismay, Prime Minister Winston Churchill’s chief-of-staff, to inform the British chiefs-of-staff, but no British or US commanders specifically invoked in 'Overlord' were informed. The British subsequently adopted a plan similar to 'Peppermint', and SHAEF assumed responsibility for co-ordinating the US and British efforts. Scientific assistance was provided by the Cavendish Laboratory in Cambridge.

'Peppermint' provided for the centralisation of all detection equipment and knowledge of its operation under ETOUSA; the establishment of a means of detecting the use of radioactive substances; and channels for the reporting of such incidents for immediate action.

Under 'Peppermint', orders were issued for medical personnel to report the details of any fogging or blackening of photographic or X-ray film, and medical officers were ordered to report diseases of unknown aetiology involving fatigue, nausea, leukopenia or erythema. Eleven survey meters and a Geiger counter were shipped to the UK early in 1944, along with 1,500 film packets, for use in measuring radiation exposure. Another 25 survey meters, five Geiger counters and 1,500 film packets were held in storage in the USA in readiness for highest-priority movement by air. Peterson instructed Chemical Warfare Service personnel in the use of the equipment, and Signal Corps personnel in its maintenance.

In the weeks immediately before the launch of the 'Neptune' (iii) amphibious assault phase that started 'Overlord', full-scale rehearsals of 'Peppermint' were undertaken to evaluate the plan and the equipment. Ground and aerial surveys were also carried out to detect the presence of radioactive substances in troop concentration areas, and at sites in the UK that had been bombed, but none was detected.

The Germans had not developed, and did not employ radioactive poisons, so 'Peppermint' was never put into effect. After Germany’s surrender in May 1945, the equipment and all the documents relating to 'Peppermint' were collected, returned to the USA and handed to the 'Manhattan Project'. However, the effort put into developing portable radiation detection equipment was not wasted: survey teams of the 'Manhattan Project' used the equipment to assess the fall-out from the 'Trinity' nuclear test in New Mexico, and the nuclear bombings of Hiroshima and Nagasaki. Together, these used 10,000 film badges.