Operation Big

Operation Big was a task force which was part of the overarching Allied effort (called Operation Alsos) to capture German nuclear secrets during the final days of World War II.

Worried that French forces might beat the US to Werner Heisenberg's laboratory in Hechingen, Boris T. Pash hastily organized a flying column of Sixth Army combat engineers ("Task Force A"). His team reached Horb three days later and headed for Haigerloch while the French troops occupied themselves with looking for members of the Vichy Government in nearby Sigmaringen.

Pash and his engineers, accompanied by General Eugene Harrison, the Sixth Army Group's Chief of Intelligence, overran Haigerloch on 23 April 1945. In a laboratory in a cellar they found a German experimental nuclear reactor whose vessel was empty of uranium and heavy water. A few drums of heavy water were later found in the laboratory's main chamber and a German scientist told Pash that the reactor's uranium cubes had been concealed beneath hay in a nearby barn.

Pash subsequently had the empty reactor blown up. The task force then proceeded to Hechingen where they found and detained Erich Bagge, Carl Friedrich von Weizsäcker, Max von Laue, and Karl Wirtz, then went on to Tailfingen where they arrested Otto Hahn. Werner Heisenberg, who had left Hechingen on 19 April, was captured by Pash and a small force at his home in Urfeld, on 3 May 1945.

Pash concluded that the German nuclear program had been years behind the Manhattan Project and that there was no possibility of them mounting any form of last-ditch nuclear attack. He called it "probably the most significant single piece of military intelligence developed during the war".