When were nuclear weapons first used in war?
Nuclear warfare
So far, two nuclear weapons have been used in the course of warfare, both by the United States near the end of World War II. On August 6, 1945, a uranium gun-type device (code name "Little Boy") was detonated over the Japanese city of Hiroshima. Three days later, on August 9, a plutonium implosion-type device (code name "Fat Man") was detonated over the Japanese city of Nagasaki. These two bombings resulted in the deaths of approximately 120,000 people.
1958 US–UK Mutual Defence Agreement
The first British atomic bomb was successfully tested in Operation Hurricane on 3 October 1952, 
but although it was more advanced than the American bombs of 1946, Britain was still several years behind in nuclear weapons technology. The United States conducted Ivy Mike, the first nuclear test of a true thermonuclear device (also known as a hydrogen bomb). The JCAE saw little benefit for the United States from sharing technology with Britain. The Soviet Union responded with the test of Joe 4, a boosted fission weapon on 12 August 1953. This prompted President Dwight Eisenhower, who was inaugurated in January 1953, to inform the United States Congress that the McMahon Act, which he considered a "terrible piece of legislation" and "one of the most deplorable incidents in American history of which he personally felt ashamed", was obsolete.
At the three-power Bermuda Conference in December 1953, Eisenhower and Churchill discussed the possibility of the United States giving Britain access to American nuclear weapons in wartime. This came to be called Project E. There were technical and legal issues that had to be overcome before American bombs could be carried in British aircraft. The United States would have to disclose their weights and dimensions, while their delivery would require data concerning their ballistics. Further down the track, there would also be issues of custody, security and targeting. The release of such information was restricted by the McMahon Act.