The UK/US plan to nuke the USSR....
The dropping of nuclear bombs on Japan was militarily unnecessary - this was widely known. I haven't heard or seen any evidence that Oppenheimer himself gave information to the Soviets - but this article hints that he did. I knew the USA was preparing to launch an all-out attack on the USSR with the connivance of the UK (christened Operation Unthinkable by Churchill) that involved a pre-emptive nuclear attack (Operation Dropshot) by the USA that the USA didn't have enough bombs to launch at the time but was building its stockpile up. It was only when the USSR succesfully tested a nuclear weapon that the plan was dropped - and so it is possible that Oppenheimer did tip the USSR them off.
'Many in Truman’s administration wished to use the atom bomb against the Soviet Union as well. President Truman, however, worried that the destruction of Moscow would lead the Red Army to invade and destroy Western Europe as a response. As such, he decided to wait until the U.S. had enough warheads to completely destroy the U.S.S.R. and its military in one fell swoop.
War planners estimated this figure to be around 400. To that end, Truman ordered the immediate ramping up of production. Such a strike, we now know, would have caused a nuclear winter that would have permanently ended all organized life on Earth.
The decision to destroy Russia was met with stiff opposition among the American scientific community. It is now widely believed that Manhattan Project scientists, including Robert J. Oppenheimer himself, passed nuclear secrets to Moscow in an effort to speed up their nuclear project and develop a deterrent to halt this doomsday scenario. This part of history, however, was left out of the 2023 biopic movie'.