One of the most critical and fascinating questions devolving from the settlement of WW II is: what role did our possession and explosion of the atomic bomb play in the formulation of US-Russian relations? The main line of opinion until now has been that, as Herbert Feis once put it, ""the light of the explosion 'brighter than a thousand suns' filtered into the conference rooms at Potsdam only as a distant gleam."" Mr. Alperovitz disagrees, and presents a convincing array of old and new evidence to support his claim that Truman, with the support of Secretary of War Stimson and Secretary of State Byrnes, shifted immediately and directly because of the bomb from FDR's ""conciliatory"" policy to a ""tough"" one intended to force ""Soviet acquiescence to American plans"" in post-war Europe. Such a theory raises very large questions regarding the accepted interpretations of the Cold War, past, present and future. One fundamental query -- ""To what extent did post-war Soviet policy derive from insecurity""-- in turn deriving from US possession of the bomb-has not been settled here because pertinent research has not yet been completed. This may well stimulate further investigation.