Answer by taking a quote from the following article:

Sir Joseph Rotblat  (4 November 1908 - 31 August 2005) was a Polish physicist, a self-described "Pole with a British passport". Rotblat worked on Tube Alloys and the Manhattan Project during World War II, but left the Los Alamos Laboratory after the war with Germany ended. His work on nuclear fallout was a major contribution toward the ratification of the 1963 Partial Nuclear Test Ban Treaty. A signatory of the 1955 Russell-Einstein Manifesto, he was secretary-general of the Pugwash Conferences on Science and World Affairs from their founding until 1973, and shared, with the Pugwash Conferences, the 1995 Nobel Peace Prize "for efforts to diminish the part played by nuclear arms in international affairs and, in the longer run, to eliminate such arms."

While still in Poland, Rotblat had realised that nuclear fission might possibly be used to produce an atomic bomb. He first thought that he should "put the whole thing out of my mind", but he continued because he thought the only way to prevent Nazi Germany from using a nuclear bomb was if Britain had one to act as a deterrent. He worked with Chadwick on Tube Alloys, the British atomic bomb project.  In February 1944, Rotblat joined the Los Alamos Laboratory as part of Chadwick's British Mission to the Manhattan Project. The usual condition for people to work on the Manhattan Project was that they had to become US citizens or British subjects. Rotblat declined, and the condition was waived. At Los Alamos, he was befriended by Stan Ulam, a fellow Polish-Jewish scientist, with whom he was able to converse in Polish. Rotblat worked in Egon Bretscher's group, investigating whether high-energy gamma rays produced by nuclear fission would interfere with the nuclear chain reaction process, and then with Robert R. Wilson's cyclotron group.  Rotblat continued to have strong reservations about the use of science to develop such a devastating weapon. In 1985 he related that at a private dinner at the Chadwicks' house at Los Alamos in March 1944, he was shocked to hear the director of the Manhattan Project, Major General Leslie R. Groves, Jr., say words to the effect that the real purpose in making the bomb was to subdue the Soviets. Historians, notably Barton Bernstein, have cast doubt on this story.  By the end of 1944 it was also apparent that Germany had abandoned the development of its own bomb and Rotblat asked to leave the project. Chadwick was then shown a security dossier in which Rotblat was accused of being a Soviet spy and that, having learned to fly at Los Alamos, he was suspected of wanting to join the Royal Air Force so that he could fly to Poland and defect to the Soviet Union. In addition, he was accused of visiting someone in Santa Fe and leaving them a blank cheque to finance the formation of a communist cell.  Rotblat was able to show that much of the information within the dossier had been fabricated. In addition, FBI records show that in 1950, Rotblat's friend in Santa Fe was tracked down in California, and she flatly denied the story; the cheque had never been cashed and had been left to pay for items not available in the UK during the war. In 1985, Rotblat recounted how a box containing "all my documents" went missing on a train ride from Washington D.C. to New York as he was leaving the country, but the presence of large numbers of Rotblat's personal papers from Los Alamos now archived at the Churchill Archives Centre "is totally at odds with Rotblat's account of events". Due to suspicions that he was a Soviet spy, Rotblat was not permitted to re-enter the United States until 1964.

Where did it take place?
Los Alamos