After 1945

Year by year

1. 1946

   Congress closes the door

   The McMahon Act, signed on 1 August, ends the sharing of atomic information with every other country, Britain included.^{Sources: 1, 13, 14}

2. 1949

   The Soviet bomb

   The Soviet Union tests its first atomic bomb at Semipalatinsk on 29 August. Truman announces it on 23 September.^{Sources: 3, 15}

3. 1952

   Britain’s bomb, and the first H-bomb

   Britain tests its own bomb in the Monte Bello Islands off Australia on 3 October. On 1 November, local time, the United States explodes Mike, the first thermonuclear device, at Enewetak.^{Sources: 5, 6, 16, 17}

4. 1954

   Castle Bravo

   A test at Bikini on 1 March yields about 15 megatons, far more than expected. Fallout lands on the Japanese fishing boat Daigo Fukuryu Maru and its 23 crew.^{Sources: 7, 8}

5. 1955

   Hiroshima hosts the world

   The first World Conference against Atomic and Hydrogen Bombs opens in Hiroshima on 6 August.^{Sources: 18}

6. 1956

   The survivors organise

   Nihon Hidankyo, the national organisation of hibakusha, is founded in Nagasaki on 10 August.^{Sources: 19, 20}

7. 1958

   The partnership restored

   Britain and the United States sign the Mutual Defence Agreement on 3 July.^{Sources: 9}

8. 1962

   The closest call

   In October the Cuban missile crisis brings the United States and the Soviet Union closest to nuclear war.^{Sources: 21}

9. 1963

   No more tests in the open air

   The Partial Test Ban Treaty, in force from 10 October, bans tests in the atmosphere, in space and under water.^{Sources: 22, 23}

10. 1967

    The American peak

    The US stockpile reaches its largest, 31,255 warheads.^{Sources: 24}

11. 1970

    The Non-Proliferation Treaty

    The treaty comes into force on 5 March.^{Sources: 25}

12. 1986

    The world’s peak

    The world holds about 70,300 nuclear warheads, counting those retired but not yet dismantled.^{Sources: 12}

13. 1996

    A test ban signed, not in force

    The Comprehensive Nuclear-Test-Ban Treaty opens for signature on 24 September. It is still not in force.^{Sources: 26}

14. 2021

    A ban on the weapons themselves

    The Treaty on the Prohibition of Nuclear Weapons comes into force on 22 January. No nuclear-armed state has joined, and nor has Japan.^{Sources: 27, 28}

15. 2024

    The survivors’ prize

    Nihon Hidankyo is awarded the Nobel Peace Prize.^{Sources: 20}

Britain and America: a promise broken

The bomb was a joint project. Britain’s MAUD report of 1941 gave the Americans a concrete plan, and a British Mission of 19 scientists worked at Los Alamos. William Penney, one of them, helped calculate the height at which the bombs should burst over Japan. ^{Sources: 29, 30, 31, 32}

In return, Roosevelt promised Churchill that the partnership would go on after the war. It did not. These are the papers that show how. ^{Sources: 33}

Britain and the bomb tells Britain’s whole part, from the Birmingham memorandum of 1940 to Polaris.

1. Date

   19 August 1943

   From

   Franklin Roosevelt and Winston Churchill

   Held at

   US National Archives (printed in FRUS) and The National Archives, Kew

   The Quebec Agreement

   we will never use this agency against each other.

   The two leaders agreed to pool “all available British and American brains and resources”, with “full and effective interchange of information and ideas” in research. Britain gave up any claim to post-war industrial advantage beyond what the President thought “fair and just”. The agreement stayed secret until Churchill published it in 1954.^{Sources: 34, 35}

   Read the agreement

2. Date

   18 September 1944

   From

   Franklin Roosevelt and Winston Churchill, initialled

   Held at

   Franklin D. Roosevelt Library, Hyde Park

   The promise at Hyde Park

   Full collaboration between the United States and the British Government in developing Tube Alloys for military and commercial purposes should continue after the defeat of Japan unless and until terminated by joint agreement.

   After Roosevelt died, American officials could not find their copy. Churchill sent a photograph of the British one in July 1945. The American copy turned up years later in the files of Roosevelt’s naval aide, apparently filed as naval supplies.^{Sources: 30, 33, 36}

   Read the aide-memoire

3. Date

   16 November 1945

   From

   Harry Truman, Clement Attlee and Mackenzie King

   Held at

   US National Archives (printed in FRUS)

   Truman renews the promise

   We desire that there should be full and effective cooperation in the field of atomic energy between the United States, the United Kingdom and Canada.

   The same day General Groves and Sir John Anderson drafted the terms. They limited “full and effective cooperation” to basic research. Building and running plants was left to arrangements to be agreed case by case.^{Sources: 37, 38}

   Read the memorandum

4. Date

   16 and 20 April 1946

   From

   Attlee to Truman, and Truman’s reply

   Held at

   US National Archives (printed in FRUS)

   Two readings of the same words

   no one at any time informed me that the memorandum was proposed with the intention of having the United States obligate itself to furnish the engineering and operation assistance necessary for the construction of another atomic energy plant. Had that been done I would not have signed the memorandum.

   Attlee wrote that “full and effective cooperation” “cannot mean less than full interchange of information and a fair division of the material.” Truman answered that the words were “very general”, and that building a British plant would undercut the hope of international control through the United Nations.^{Sources: 30, 39, 40}

   How it is read

   • The official US history calls the American position “manifestly unfair”, but says the hope of international control “played the major part” in Truman’s thinking.

   Read Truman’s letter

5. Date

   1 August 1946

   From

   The US Congress, signed by President Truman

   Held at

   US Statutes at Large, 60 Stat. 755

   The McMahon Act

   there shall be no exchange of information with other nations with respect to the use of atomic energy for industrial purposes

   The Atomic Energy Act did not name Britain, but its controls on “restricted data” ended the exchange with every country, Britain included. British scientists at Los Alamos lost access even to their own reports. The last of them left in April 1947.^{Sources: 1, 2, 13, 31}

   Read the Act

6. Date

   1952, read to the Commons on 5 April 1954

   From

   Senator Brien McMahon, quoted by Churchill

   Held at

   Hansard, House of Commons, 5 April 1954

   The senator who did not know

   the nature of the agreements which made this contribution possible was not disclosed to me and my colleagues on the Senate Special Atomic Energy Committee at the time we framed the law in 1946.

   In the same debate Churchill blamed the Labour government for not pressing the agreements. Attlee replied: “We did not abandon anything.” The United States administration, he said, had been “prevented by the action of the Senate”.^{Sources: 35, 41}

   Read the debate

7. Date

   3 July 1958

   From

   The governments of the United Kingdom and the United States

   Held at

   The National Archives and GOV.UK Treaty Series

   The partnership restored

   the concept of national self-sufficiency is now out of date.

   After Sputnik, Eisenhower and Macmillan declared that interdependence in October 1957. Congress then allowed sharing with a nation that “has made substantial progress in the development of atomic weapons”. The agreement was signed on 3 July 1958. Since 14 November 2024 it has had no end date.^{Sources: 9, 42, 43, 44, 45}

   Read the 1957 declaration

The American side had its reasons. The official US history names wartime distrust, security, a Congress set on protecting the secret, and the arrest in 1946 of Alan Nunn May, a British scientist who had passed information to the Soviet Union. ^{Sources: 30, 46}

Above all, it says, Truman did not want a close tie with Britain to spoil the hope of international control. ^{Sources: 30, 40}

Britain goes alone

On 8 January 1947 a small committee of ministers decided that Britain would build its own bomb, in secret. Parliament was told in May 1948. ^{Sources: 5, 13, 47}

The Foreign Secretary, Ernest Bevin, is remembered as saying “We’ve got to have the bloody Union Jack on top of it”, but that comes from a recollection in 1982; the minutes of the meeting record his views “in a more measured way”. ^{Sources: 13}

Klaus Fuchs, a member of the British Mission at Los Alamos, had been passing atomic secrets to the Soviet Union since 1941. He was arrested in Britain on 2 February 1950 and sentenced to 14 years. His arrest halted the talks that were trying to restore cooperation. ^{Sources: 48, 49}

Britain tested its first bomb, Operation Hurricane, on 3 October 1952. It was exploded inside the hull of the frigate HMS Plym in the Monte Bello Islands, off Western Australia, and yielded 25 kilotons. William Penney led the test. ^{Sources: 5, 6, 10, 14}

Britain made 12 atomic tests in Australia between 1952 and 1957. An Australian Royal Commission reported in 1985 that the Monte Bello Islands “were not an appropriate place for atomic tests”, and that the Hurricane trial failed “to consider the distinctive lifestyles of Aboriginal people”. ^{Sources: 6, 50}

The Soviet bomb

The Soviet Union tested its first bomb, RDS-1, at Semipalatinsk on 29 August 1949. It was a plutonium implosion bomb, closely based on the American design that Fuchs and others had passed to Moscow. The Atomic Heritage Foundation estimates that spying saved the Soviet programme between six months and two years. ^{Sources: 3, 51}

An American weather aircraft, a WB-29, picked up its radioactive debris over the Pacific on 3 September. On 23 September Truman told the world: “We have evidence that within recent weeks an atomic explosion occurred in the U.S.S.R.” ^{Sources: 15, 52}

The hydrogen bomb and the Lucky Dragon

In January 1950 Truman ordered work on the hydrogen bomb. The first American thermonuclear device, Mike, weighed 82 tons and yielded 10.4 megatons at Enewetak on 1 November 1952, local time. The Soviet Union’s first megaton-range hydrogen bomb followed on 22 November 1955. ^{Sources: 16, 53, 54, 55}

On 1 March 1954 the American test Castle Bravo, at Bikini, yielded about 15 megatons, nearly three times what was expected. Fallout fell on islands far downwind and on the Japanese tuna boat Daigo Fukuryu Maru. All 23 of its crew were exposed. ^{Sources: 7, 8}

The boat’s chief radio operator, Aikichi Kuboyama, died on 23 September 1954. The cause of his death is disputed; the US National Security Archive describes it as liver ailments “possibly aggravated” by his exposure. ^{Sources: 7, 56}

In Japan a petition against the bombs gathered more than 20 million signatures by the end of 1954. ^{Sources: 57}

It led to the first World Conference against Atomic and Hydrogen Bombs, in Hiroshima in August 1955, and to the founding of Nihon Hidankyo in Nagasaki in August 1956. Its founders included victims of the Pacific tests as well as the survivors of Hiroshima and Nagasaki. ^{Sources: 18, 19, 20, 57}

The B-29 after the war

The 509th Composite Group left Tinian on 17 October 1945 and went home to Roswell, New Mexico. It later became part of the new Strategic Air Command. ^{Sources: 58, 59}

B-29s went back to war over Korea in 1950. Soviet-built MiG-15 jet fighters proved deadly to them: on one day in October 1951 MiGs knocked out six of nine B-29s, and from that November they bombed only at night. Strategic Air Command retired its last B-29 bomber on 4 November 1954. ^{Sources: 60, 61, 62}

The Soviet Union built its own B-29. Three American B-29s had landed in the Soviet Far East in 1944 and were interned. Stalin ordered an exact copy. The result, the Tu-4, first flew in May 1947, and on 18 October 1951 a Tu-4 dropped a Soviet atomic bomb. ^{Sources: 63}

The arms race in numbers

Figures for warheads change. Each is given with the year it was true. ^{Sources: 12}

More than 2,000 nuclear tests were carried out between 1945 and 1996: 1,032 by the United States, 715 by the Soviet Union, 210 by France, 45 by Britain, 45 by China and one by India. Ten more have followed since 1996, by India, Pakistan and North Korea. ^{Sources: 4}

The treaties

The test ban treaty cannot come into force until all 44 states named in it have ratified. China, Egypt, Iran, Israel and the United States have signed but not ratified; India, Pakistan and North Korea have not signed; Russia withdrew its ratification in 2023. ^{Sources: 26, 65}

Japan has not joined the prohibition treaty. Its government says no nuclear-armed state is likely to join, given the treaty’s “incompatibility with nuclear deterrence”, and prefers to work through the Non-Proliferation Treaty. ^{Sources: 28, 66}

Today

Nine countries hold nuclear weapons: the United States, Russia, Britain, France, China, India, Pakistan, North Korea and Israel. In January 2026 they held an estimated 12,187 warheads, about 4,012 of them deployed with missiles and aircraft. ^{Sources: 11, 12}

The United States and Russia hold about 86 per cent of them. ^{Sources: 12}

No nuclear weapon has been used in war since Nagasaki. ^{Sources: 16, 20, 67}