The US atempted to prevent Britain developing nuclear weapons by withdrawing all co-operation.
They only changed their minds when it was clear Britain was doing it independently anyway
Britain and Canada rolled all its nuclear efforts in to the Manhattan project during the war on the understanding that the technology would be shared.
The US decided they weren't going to do that.
Clement Attlee was fairly outraged and decided Britain would do it alone (which it did) and the US only decided to share after it was clear we were oing it without them.
Had Britain not developed our own nukes the US would have continued to withhold all co-operation
Brief Wiki synopsis-
End of American cooperation
President
Harry Truman and prime ministers
Clement Attlee and
Mackenzie King boarding
USS Sequoia for discussions about nuclear weapons, November 1945
With the end of the war, the
Special Relationship between Britain and the United States "became very much less special".
[42] The British government had trusted that America would share nuclear technology, which it considered a joint discovery.
[43] On 8 August 1945 the Prime Minister,
Clement Attlee, sent a message to President
Harry Truman in which he referred to themselves as "heads of the Governments which have control of this great force".
[43] But Roosevelt had died on 12 April 1945, and the Hyde Park Agreement was not binding on subsequent administrations.
[44] In fact, it was physically lost. When Wilson raised the matter in a Combined Policy Committee meeting in June, the American copy could not be found.
[45]
On 9 November 1945, Attlee and the
Prime Minister of Canada,
Mackenzie King, went to Washington, D.C., to confer with Truman about future cooperation in nuclear weapons and nuclear power.
[46][47] A Memorandum of Intention they signed replaced the Quebec Agreement. It made Canada a full partner, continued the Combined Policy Committee and Combined Development Trust, and reduced the obligation to obtain consent for the use of nuclear weapons to merely requiring consultation.
[48] The three leaders agreed that there would be full and effective cooperation on atomic energy, but British hopes were soon disappointed;
[49] the Americans restricted cooperation to basic scientific research.
[50]
The next meeting of the Combined Policy Committee on 15 April 1946 produced no accord on collaboration, and resulted in an exchange of cables between Truman and Attlee. Truman cabled on 20 April that he did not see the communiqué he had signed as obligating the United States to assist Britain in designing, constructing and operating an atomic energy plant.
[51] The passing of the
Atomic Energy Act of 1946 (McMahon Act) in August 1946, which was signed by Truman on 1 August 1946, and went into effect at midnight on 1 January 1947,
[52] ended technical cooperation. Its control of "restricted data" prevented the United States' allies from receiving any information.
[53] The remaining British scientists working in the United States were denied access to papers that they had written just days before.
[54]
This partly resulted from the arrest for espionage of British physicist
Alan Nunn May, who had worked in the Montreal Laboratory, in February 1946, while the legislation was being debated.
[55] It was but the first of a series of spy scandals. The arrest of Klaus Fuchs in January 1950,
[56] and the June 1951 defection of
Donald Maclean, who had served as a British member of the Combined Policy Committee from January 1947 to August 1948, left Americans with a distrust of British security arrangements