Even when the USSR was technically an ally against fascism, the USA & the UK conspired to monopolize the atomic bombs, with a view toward strengthening their hands against the USSR & communists everywhere in the post-WW2 world. For men like Churchill, fighting against fascists was often secondary to their anticommunism. Martin J. Shervin writes in "The Atomic Bomb and the Origins of the Cold War," _Origins of the Cold War: An International History_, eds. Melvyn P. Leffler & David S. Painter (NY: Routledge, 1994):
***** Convinced that wartime atomic energy policies would have postwar diplomatic consequences, several scientists advised Roosevelt to adopt policies aimed at achieving a postwar international control system. Churchill, on the other hand, urged the President to maintain the Anglo-American atomic monopoly as a diplomatic counter against the postwar ambitions of other nations -- particularly against the Soviet Union. Roosevelt fashioned his atomic energy policy from the choices he made between these conflicting recommendations. In 1943 he rejected the counsel of his science advisers and began to consider the diplomatic component of atomic energy policy in consultation with Churchill alone....[H]e pursued policies consistent with Churchill's monopolistic, anti-Soviet views....
...Roosevelt was perfectly comfortable with the concept Churchill advocated -- that military power was a prerequisite to successful postwar diplomacy. As early as August 1941, during the Atlantic Conference, Roosevelt had rejected the idea that an "effective international organization" could be relied upon to keep the peace: an Anglo-American international police force would be far more effective, he told Churchill.[18] By the spring of 1942 the concept had broadened: the two "policemen" became four, and the idea was added that every other nation would be totally disarmed. "The Four Policemen" would have "to build up a reservoir of force so powerful that no aggressor would dare to challenge it," Roosevelt told Arthur Sweetser, an ardent internationalist. Violators first would be quarantined, and if they persisted in their disruptive activities, bombed at the rate of a city a day until they agreed to behave. A year later, at the Teheran Conference, Roosevelt again discussed his idea, this time with Stalin. As Robert A. Divine has noted: "Roosevelt's concept of big power domination remained the central idea in his approach to international organization throughout World War II."[19]
...If Roosevelt was less worried than Churchill about Soviet postwar ambitions, he was no less determined than the Prime Minister to avoid any commitments to the Soviets for the international control of atomic energy. There could still be four policemen, but only two of them would have the bomb.
...Had Roosevelt avoided all postwar atomic energy commitments, his lack of support for international control could have been interpreted as an attempt to reserve his opinion on the best course to follow. But he had made commitments in 1943 supporting Churchill's monopolistic, anti-Soviet position, and he continued to make others in 1944. On June 13, for example, Roosevelt and Churchill signed an Agreement and Declaration of Trust, specifying that the United States and Great Britain would cooperate in seeking to control available supplies of uranium and thorium ore both during and after the war.[21] This commitment, taken against the background of Roosevelt's peacekeeping ideas and his other commitments, suggests that the President's attitude toward the international control of atomic energy was similar to the Prime Minister's.
...By 1944 Roosevelt's earlier musings about the Four Policemen had faded into the background. But the idea behind it, the concept of controlling the peace of the world by amassing overwhelming military power, appears to have remained a prominent feature of his postwar plans....
[18] _Foreign Relations of the United States_, 1941 (Washington, DC, 1958), Vol. 1: 363, 365-6. [19] Roosevelt, quoted in "Mr Sweester's Notes," May 29, 1942, Arthur Sweester Papers, box 39, Library of Congress. See also _FRUS: The Conferences at Cairo and Teheran, 1943_ (Washington, DC, 1961), 530-2, and Robert A. Divine, _Roosevelt and World War II_ (Baltimore, MD, 1970), 58. [21] Agreement and Declaration of Trust, June 13, 1944, in Leslie R. Groves, "Diplomatic History of the Manhattan Project," annex 22a, in Manhattan Engineer District Files, National Archive; also in Margaret Gowing, _Britain and Atomic Energy_ (London, 1964), pt 1, app. 1. *****
Atomic secrecy was an index of the Anglo-American desire to dominate the post-WW2 world. Unlike silly liberals who think in terms of "whether or not Rosenberg was a spy" (and think that if he was, he was indeed "guilty"), men like Churchill, Roosevelt, Truman were thinking in terms of who controls the world. And they had no qualms about brandishing the Bombs to cow other nations and revolutionary movements into submission in the post-WW2 world. That was and is their idea of "peace."
Yoshie