A dark secret behind the Hiroshima bomb is where the uranium came from, a spy-vs.-spy race to secure naturally enriched uranium from Congo to fuel the Manhattan Project and keep the rare mineral out of Nazi hands, reports Joe Lauria.

Since the first use of a nuclear weapon in Hiroshima 71 years ago, on Aug. 6, 1945, the story of where the uranium for the bomb came from and the covert operation the U.S. employed to secure it has been little known.