The reason the US broke off its collaboration with the UK was because Congress passed the Atomic Energy Act of 1946 (the McMahon Act) which made the transference of any classified information about nuclear weapons to any other countries illegal. This aspect was added in the last moments before the bill was brought to vote, as the Congressmen reeled from the first of several "atomic spy" scandals (the Gouzenko affair, which was leaked to the press by pro-secrecy factions in Washington with this specific goal in mind). The Congressmen were ignorant of things like the Quebec Agreement which in short made the UK an "equal partner" in information sharing (because it was secret). They lamented this afterwards.

Truman had little to do with it (he had little to do with the content of the Atomic Energy Act) and in my mind it had nothing to do with international control, which Truman was fairly lukewarm about anyway, and sharing this kind of information would not have deeply complicated it (the UK already knew how to make bombs, and the international control schemes in question were about controlling material things, not ideas — factories and ore are relatively easy to control, while equations and drawings are not). The bit about international control being relevant seems like an interpretation from afar, not based on what was going on at the time. One finds a lot of this even in the historical literature, alas — people try to read various "reasons" into legislation because their either didn't have access to or didn't look for the actual work that went into passing it.

(I have gone over many drafts of the AEA and the statements of the people who crafted its various components quite closely. Many aspects of it were hastily written and frankly look inexperienced and poorly thought-out. The chief architect of the legislation, Brien McMahon, was a junior Senator who believed that if you deliberately wrote contradictory things into laws, then reasoned debate would occur over how to interpret them and rationality would win out. This is not intelligent legislative practice. The AEA was a mess in many ways, and recognized as such as at the time, but signed into being because no good alternatives existed and in its absence the US nuclear infrastructure built up during the Manhattan Project was falling apart.)

As for consequences: It did strain things. The UK decided in 1947 to develop its own nuclear weapon as a direct result of the severing of contact. The US weapons workers and military did desire to share some "restricted data" with the UK, both because they believed that the UK's scientists had much to offer, but also because they planned to use the UK as one of their forward bases in Europe. Also, the UK was still a member of the Combined Development Trust, which meant that they still controlled a significant fraction of the world's uranium resources. This meant that the US did need their cooperation and aid, because the US was desperately afraid of not having enough raw uranium to expand their stockpile in the ways they wanted to do. So they could not really just cut the British out — it just meant that their relationship with the British was hampered and made more awkward.

When it came time to revise the McMahon act in 1954, weakening this provision was one of the key reforms. By this time of course the UK already had the bomb. By 1958 the US and UK had signed an agreement to collaborate closely. The apparent slowness of all of this return to cooperation owed in no small part to the revelation that the UK's scientists and government had been deeply penetrated by Soviet spies, which came out in 1950, at a point when the US was once again considering opening up connections with the UK.

Follow-up question: did this also happen in other areas of science and engineering? I remember a documentary on Discovery about the post-war jet programs in Britain and USA, and breaking the sound barrier. Some of the British engineers interviewed complained that the Americans came and looked at everything they had, but when the time came for the British to visit the Americans, it never happened. Is this accurate? How widespread was this?

The Acheson-Lilienthal report and Baruch plans are relevant.

A special advisory commission under [Dean Acheson and David Lilienthal was commissioned to prepare a proposal/report to the United Nations Atomic Energy Commision (UNAEC) Robert Oppenheimer was pulled in for scientific advice and provided the key insight of monitoring the mining..

The report called for creating an Atomic Development Authority for overseeing the mining and use, operating plants that could contribute to weapons and licensing peaceful research. The US would give up its monopoly on weapons (no timeline was set) and reveal info to the Soviet Union in exchange for a mutual agreement that no more weapons would be created. The plan was intended to be a base for discussion and not a final fixed proposal.

However just before submission to the UN, Truman appointed Bernard Baruch as the US delegate to the UNAEC. He presented his own plan which borrowed heavily from the Acheson Lilenthal report but added enforcement with penalties that could not be vetoed by the Security Council.

Unrestricted international inspection of Soviet domestic facilities also turned out to be a sticking point.

The Soviet Union felt the Security Council was already stacked against them and giving up their veto was unacceptable, as was unrestricted international inspection of Soviet domestic facilities; another issue was that the US was to start destroying its arsenal only after the plan was fully implemented.

The Soviet Union presented the Gromyko Plan in response ; a convention was to completely dismantle production and possession of nuclear weapons and only then would negotiations on adherence commence.

The UNAEC voted 10-0 on the Baruch Plan on December 30 1946, but the Soviet Union and Poland abstained.

Though the plan continued to be discussed through 1948, it was essentially dead in the water.

The US side did not consider any change in the ADA or the staged implementation of exchange and control; both the US and the USSR continued work on nuclear weapons. Hostility and mistrust increased; the Berlin airlift in June 1948 and the Soviet bomb in 1949 followed.

The failure of the Baruch Plan virtually guaranteed the nuclear arms race.