Allied technological cooperation during World War II
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The Second World War was not won by one nation; the Allies had to cooperate while fighting on the ground, as well as by sharing technological resources and innovations. There are various ways in which the allies cooperated, including the American lend lease scheme and hybrid weapons such as the Sherman Firefly.
The origin of the cooperation stemmed from a 1940 visit by the Aeronautical Research Committee chairman Henry Tizard that arranged to transfer UK military technology to the US in case of the successful invasion of the UK that Hitler was planning as Operation Sea Lion. Tizard led a British technical mission, containing details and examples of British technological developments in fields such as radar, jet propulsion and also the early British research into the Atomic bomb. One of the devices brought to the US by the Mission, the resonant cavity magnetron, was later described as "the most valuable cargo ever brought to our shores".
Small arms 
Small arms really began to be shared after the fall of France, most of the ‘sharing’ being one sided as America was not yet directly involved in the conflict and thus all the movement of food was from the United States to the United Kingdom. Although in the months following Operation Dynamo British manufacturers progressed in making good the material lost by the British Army in France, the British government looked overseas for additional sources of equipment to assist in overcoming shortages and prepare for future offensives. The most extreme example of the shortages were found in the quickly improvised Local Defence Volunteers, later renamed the Home Guard, who were forced to train with broom handles and makeshift pikes using lengths of piping and old bayonets until proper weapons could be supplied.
In addition to those produced in Britain, small arms and ammunition were obtained from Commonwealth countries and also purchased from U.S. manufacturers until supplied under lend lease in 1941. The weapons obtained from America for the British army were the famous Tommy gun, M1911A1 handgun and the Colt .45 revolver. The Home Guard received Browning .30 machine guns in small numbers, M1918 .30 BARs again in limited numbers and P17 .30 Enfield Rifles (the British .303 P14 version also being issued).
Slightly later in the war the M1919 .30 Machinegun (commonly known as the 30-cal) and the M2HB .50 Machine gun entered service with British Infantry and as anti-aircraft machine guns on British tanks and notably with the SAS on their heavily armed Jeeps. The Browning had been standard fitment on British aircraft since the late 1930s
Britain did supply small arms to the USSR, the ubiquitous 9mm Sten Submachine gun was supplied to Soviet partisan troops.
The British made use of many American towed artillery pieces during the war, such as the M2 105 mm howitzers, M1A1 75mm Pack Howitzers, 155 mm guns (Long Toms). These weapons were again supplied under lend-lease or bought outright. Tank/tank destroyer guns used by the British included the 37 mm M5/M6 Gun (General Stuart and General Grant/Lee tanks), 75mm M2 Gun (General Grant/Lee), 75 mm M3 Gun (General Grant/Lee and General Sherman), 76 mm Gun M1 (General Sherman) and 3" Gun M7 (3in SP, Wolverine).
The Americans in turn did use a British artillery piece, the Ordnance QF 6-pounder 7cwt anti-tank gun. The U.S realized at the start of the war that their own 37 mm Gun M3 would soon be obsolete and thus they produced a license built version of the QF 6-pounder under the designation 57 mm Gun M1.
Both 76 mm and 75 mm guns were mounted on tanks sent to the Soviets by the U.S and the British tanks sent were armed with both the Ordnance QF 2-pounder and the Ordnance QF 6-pounder.
Another technology taken to the US, by Tizard, for further development and mass production, was the (radio-frequency) proximity fuse.
Tanks and other vehicles 
Probably the most well-known tank of the Second World War in Anglophone countries is the Medium Tank M4. Used in all theaters it was a versatile reliable design and easy to produce, thus huge numbers were made and huge numbers were provided to both Britain and the USSR under lend-lease. Despite official opinions the Medium Tank M4 was well liked by some Soviet tankers, while others called it the best tank for peacetime service. When Britain received the tank, it was given the designation Sherman and this is where the name of the Sherman tank came from - the U.K. naming its U.S.-built tanks after American Civil War generals. Both the British and the Soviets re-armed their M4s with their own tank guns. The Soviets re-armed a very small number with the standard 76 mm F-34 tank gun but so much 75 mm ammunition was supplied by the USA that the conversions were not widespread. The British conversion is better known as the Sherman Firefly, mounting the potent Ordnance QF 17-pounder anti-tank gun in the standard turret with a new mantlet and the radio moved to an armoured box welded to the turret bustle. These conversions both show how a blend of both countries' weaponry was desirable, though unfortunately despite the Americans building a few 17-pounder Fireflies from new, it never went into mass production and did not see action. The Firefly thus remains a purely British variant of the Sherman. The M10 Tank Destroyer, which in British service was named Wolverine, was also up-gunned with the 17-pounder, creating the highly potent 17pdr SP Achilles tank destroyer. This was used in accordance with British tactical doctrine for tank destroyers, in that they were considered self-propelled anti-tank guns rather than aggressive 'tank hunters'. Used in this fashion, it was a highly effective weapon.
The British also used the Sherman hull for two other Sherman variants known as:
The DD was a swimming tank, a flotation screen gave buoyancy and two propellers powered by the tank's engine gave propulsion in the water. On reaching land the screens could be dropped and the tank could fight in the normal manner. The DD was used by both British and American forces during Operation Overlord. The DD had impressed U.S. General Dwight D. Eisenhower during demonstrations and was readily accepted. It was another key example of combining technologies. Unfortunately for the Americans they did not accept the Crab which could have saved many engineers' lives by clearing mines under fire protected by armour. Armoured recovery vehicles (ARVs) were also converted from Shermans by the British as well as the specialist BARV (Beach Armoured Recovery Vehicle) designed to push-off landing craft and salvage vehicles which would otherwise have been lost.
The British also supplied tanks to the USSR in the form of the Matilda, Valentine and Churchill infantry tanks. Soviet tank soldiers liked the Valentine for its reliability, cross country performance and low silhouette. The opinion of the Matilda and Churchill was quite low considering their weak 40-mm guns (without HE shells) and inability to operate in harsh winter and offroad conditions.
Deliveries of M3 half-tracks from the USA to the USSR were a major benefit to mechanized Red Army units. Soviet industry produced few armoured personnel carriers, so lend-lease American vehicles were in great demand for fast movement of troops in front-line conditions. While M3s had only limited protection, common trucks had no protection at all. Moreover, a large part of the Red Army truck fleet was American Studebakers, which were highly regarded by Soviet drivers. After the war, Soviet designers paid a lot of attention to create their own 6x6 army truck and the Studebaker was the template for this development.
In addition to being originally designed to a British specification for use by the RAF, the famous P-51 Mustang would not have been the excellent fighter that it was without British technology in the form of the equally famous Rolls-Royce Merlin aero-engine. It is another excellent example of technological cooperation, in this case American airframe, British engine. The P-51 was also used in small numbers by the British. In addition to the British making use of American planes the U.S also made use of some Supermarine Spitfires based in the U.K and Mediterranean, as well as using Bristol Beaufighters in the Mediterranean, and de Havilland Mosquitoes based in the U.K.
Nuclear research 
In 1942, and with the threat of invasion by Germany still apparent, the United Kingdom dispatched around 20 British scientists and technical staff to America, along with their work, which had been carried out under the codename Tube Alloys, to prevent the potential for vital information falling into enemy hands. The scientists joined the Manhattan Project at Los Alamos, New Mexico, where their work on uranium enrichment was instrumental in jump-starting the project. This collaboration eventually led to the Mutual Defence Agreement between the two nations, whereby American nuclear weapons technology was adapted for British use.
Code-breaking technology 
Considerable information was transmitted from the UK to the US during and after WWII relating to code-breaking methods, the codes themselves, cryptoanalyst visits, mechanical and digital devices for speeding code-breaking etc. When the Atlantic convoys of war materiel from the US to the UK came under serious threat from U-boats, considerable encouragement and practical help was given by the US to accelerating the development of code-breaking machines.
Subsequently, the co-operation led to significant success in Australia and the far East at breaking encrypted Japanese messages.
See also 
- ASDIC (sonar)
- Bletchley Park
- British Purchasing Commission
- Cavity magnetron
- Frank Whittle
- Gyro gunsight
- History of radar
- Jet engine
- Lend-Lease Sherman tanks
- Liberty ship
- List of World War II electronic warfare equipment
- Manhattan Project
- M4 Sherman variants
- Operations research
- Radiation Laboratory
- Telecommunications Research Establishment
- Tizard Mission
- Traveling-wave tube
- Proximity fuze
- James Phinney Baxter III (Official Historian of the Office of Scientific Research and Development), Scientists Against Time (Boston: Little, Brown, and Co., 1946), page 142.
- Boris Kavalerchik, Voenno-Istoricheskiy Arkhiv, issue No. 1, 2006