In February 1944, Rotblat joined the Los Alamos Laboratory as part of Chadwick's British Mission to the Manhattan Project. Although he was upset by the morality of the project, he believed the allies needed to be able to threaten retaliation in case Germany developed the bomb. 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.