Editing Manhattan Project (section)

=== Collaboration with the United Kingdom ===
{{Main|British contribution to the Manhattan Project}}
The British and Americans exchanged nuclear information but did not initially combine their efforts; in 1940–41 the British project ([[Tube Alloys]]) was larger and more advanced.{{sfn|Phelps|2010|pp=282–283}} British leadership initially opposed an offer by Bush and Conant in August 1941 to pool the nations' atomic efforts,{{sfn|Farmelo|2013|pp=188–195}} but the British, who had made significant advances in research early in the war, did not have the resources to carry it into development while devoting a large portion of their economy to the war; Tube Alloys soon fell behind its American counterpart.{{sfn|Bernstein|1976|pp=206–208}} The roles of the two countries were reversed,{{sfn|Villa|1981|pp=144–145}} and in January 1943 Conant notified the British that they would no longer receive atomic information except in certain areas.{{sfn|Stacey|1970|p=517}}{{sfn|Bernstein|1976|p=211}} The British investigated the possibility of an independent nuclear program but determined that it could not be ready in time to impact the [[European Theatre of World War II|war in Europe]].<ref name="fakley1983">{{cite journal |url=http://www.atomicarchive.com/History/british/index.shtml |title=The British Mission |author=Fakley, Dennis C. |journal=Los Alamos Science |date=Winter–Spring 1983 |issue=7 |pages=186–189}}</ref>

[[File:Groves and Chadwick 830308.jpg|thumb|left|Groves confers with [[James Chadwick]], the head of the British Mission.|alt=A large man in uniform and a bespectacled thin man in a suit and tie sit at a desk.]]
By March 1943 Conant decided that [[James Chadwick]] and one or two other British scientists were important enough that the bomb design team at Los Alamos needed them, despite the risk of revealing weapon design secrets.{{sfn|Bernstein|1976|pp=213}} In August 1943 Churchill and Roosevelt negotiated the [[Quebec Agreement]],{{sfn|Gowing|1964|pp=168–173}}{{sfn|Bernstein|1976|pp=216–217}} which established the [[Combined Policy Committee]] to coordinate the efforts of the US and UK; Canada was not a signatory, but the Agreement provided for a Canadian representative on the Combined Policy Committee in view of Canada's contribution to the effort.{{sfn|Jones|1985|p=296}} An agreement between Roosevelt and Churchill known as the [[Hyde Park Aide-Mémoire]], signed in late September 1944, extended the Quebec Agreement to the postwar period and suggested that "when a 'bomb' is finally available, it might perhaps, after mature consideration, be used against the Japanese, who should be warned that this bombardment will be repeated until they surrender".<ref>{{cite web|url=https://ahf.nuclearmuseum.org/ahf/key-documents/hyde-park-aide-memoire/|title=Hyde Park Aide-Mémoire (18 September 1944)|date=2022|publisher=Atomic Heritage Foundation}}</ref>{{sfnb|Gowing|1964|pp=340–342}}

When cooperation resumed after the Quebec Agreement, the Americans' progress and expenditures amazed the British. Chadwick pressed for British involvement in the Manhattan Project to the fullest extent and abandoned hopes of an independent British project during the war.{{r|fakley1983}} With Churchill's backing, he attempted to ensure that every request from Groves for assistance was honored.{{sfn|Gowing|1964|pp=242–244}} The British Mission that arrived in the United States in December 1943 included [[Niels Bohr]], Otto Frisch, [[Klaus Fuchs]], Rudolf Peierls, and [[Ernest William Titterton|Ernest Titterton]].{{sfn|Hunner|2004|p=26}} More scientists arrived in early 1944. While those assigned to gaseous diffusion left by the fall of 1944, the thirty-five working under Oliphant with Lawrence at Berkeley were assigned to existing laboratory groups and most stayed until the end of the war. The nineteen sent to Los Alamos also joined existing groups, primarily related to implosion and bomb assembly, but not the plutonium-related ones.{{r|fakley1983}} The Quebec Agreement specified that nuclear weapons would not be used against another country without the mutual consent of the US and UK. In June 1945, Wilson agreed that the nuclear bombing of Japan would be recorded as a decision of the Combined Policy Committee.{{sfn|Gowing|1964|p=372}}

The Combined Policy Committee created the [[Combined Development Trust]] in June 1944, with Groves as its chairman, to procure uranium and [[thorium#Occurrence|thorium ores]] on international markets. The [[Belgian Congo]] and Canada held much of the world's uranium outside Eastern Europe, and the [[Belgian Government in Exile]] was in London. Britain agreed to give the United States most of the Belgian ore, as it could not use most of the supply without restricted American research.{{sfn|Bernstein|1976|pp=223–224}} In 1944, the Trust purchased {{convert|3440000|lb|kg}} of uranium oxide ore from companies operating mines in the Belgian Congo. To avoid briefing US Secretary of the Treasury [[Henry Morgenthau Jr.]], a special account not subject to the usual auditing and controls was used to hold Trust monies. Between 1944 and his resignation from the Trust in 1947, Groves deposited a total of $37.5&nbsp;million.{{sfn|Jones|1985|pp=90, 299–306}}

Groves later said that the British scientists' direct contributions to the Manhattan Project were "helpful but not vital," but "there probably would have been no atomic bomb to drop on Hiroshima" without Britain's (particularly Churchill's) impetus.{{sfn|Groves|1962|p=408}} The British wartime participation was crucial to the success of their [[High Explosive Research|independent nuclear weapons program]] when the [[McMahon Act]] of 1946 temporarily ended American nuclear cooperation.{{r|fakley1983}}