The Allies of World War II cooperated extensively in the development and manufacture of new and existing technologies to support military operations and intelligence gathering during the Second World War. There are various ways in which the allies cooperated, including the American Lend-Lease scheme and hybrid weapons such as the Sherman Firefly as well as the British Tube Alloys nuclear weapons research project which was absorbed into the American-led Manhattan Project. Several technologies invented in Britain proved critical to the military and were widely manufactured by the Allies during the Second World War.
Tizard Mission
The origin of the cooperation stemmed from a 1940 visit by the Aeronautical Research Committee chairman Henry Tizard, during which Tizard arranged to transfer UK military technology to the US in the event that Hitler's planned invasion of the UK should succeed. Tizard led a British technical mission, known as the Tizard 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 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 was from the United States to the United Kingdom. In the months following Operation Dynamo, as British manufacturers progressed in building replacements for the materiel 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 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 they were supplied under Lend-Lease beginning in 1941. The weapons obtained from the United States included the Tommy gun, M1911A1 pistol and the M1917 revolver produced by Colt and Smith & Wesson, all primarily produced in .45 ACP. The Home Guard received the Browning .30 machine gun, the M1918 .30 BAR and the P17 .30 Enfield rifle. M1917 Enfield rifles chambered for .303 British were also provided by the U.S. while all .30-caliber U.S. rifles, BARs and machine guns were chambered for .30-06 Springfield
Later, the M1919 .30 machine gun and the M2HB .50 machine gun chambered in .50 BMG were provided by the U.S. for infantry and anti-aircraft use. Browning AN2 light machine guns in .303 British caliber were already in standard use on British aircraft beginning in the late 1930s.
Britain supplied small arms to the USSR, and the 9mm Sten submachine gun was supplied to Soviet partisan troops.
Artillery
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 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 M10).
The Americans in turn used a British artillery piece, the Ordnance QF 6-pounder 7cwt anti-tank gun. The US 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 US, while 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 Henry Tizard, for further development and mass production, was the (radio-frequency) proximity fuse. It was five times as effective as contact or timed fuzes and was devastating in naval use against Japanese aircraft and so effective against German ground troops that General George S. Patton said it "won the Battle of the Bulge for us."