Colonel John Lansdale, Jr., was a civilian lawyer and Army reservist who requested a call to active duty with the War Department's Military Intelligence Division. He served as the Director of Intelligence and Security for the Manhattan Project from 1941 to 1946.

The United States program to develop the atomic bomb began in August 1942. From the beginning, the need for security was recognized as paramount. The project had to be protected from sabotage and espionage and, equally important, the fact that the US was working on such a program had to be kept under wraps at all cost. Early on, a Protective Security Section (PSS) was established for personnel and information security, facility protection and security education. Maj. John A. Lansdale, of the Military Intelligence Division (MID), acted as a liaison between the PSS and the Assistant Chief of Staff, Intelligence on the War Department General Staff.

By February 1943, a more comprehensive counterintelligence program was warranted and Counter Intelligence Corps (CIC) agents Capt. Horace K. Calvert and Capt. Robert J. McLeod were assigned to the Manhattan Engineer District (MED) to organize the Intelligence Section. More CIC personnel followed, with agents stationed at the Clinton Engineer Works, Chicago, St. Louis, Site Y (Los Alamos, New Mexico), and Berkeley, California. By August 1943, the Intelligence Section merged with the PSS and moved to Oak Ridge, Tennessee. At this time, it assumed responsibility for every aspect of security within the MED. Finally, on December 18, 1943, a CIC Detachment was organized specifically for this work. The Detachment was comprised of 25 officers and 137 enlisted agents, each one hand-picked by Captains Calvert and McLeod. The final piece fell into place on January 7, 1944, when now Lt. Col. Lansdale transferred from the MID to the CIC Detachment to become special assistant to General Leslie Groves, the Chief of the MED. Lansdale assumed full responsibility for all intelligence and security matters affecting the district. In addition to preventing unintentional disclosure of information and infiltration by enemy agents, Lansdale's operation included preventing fires and explosions, monitoring courier duties, protecting classified shipments, educating personnel about security importance and measures, obtaining newspaper cooperation, and conducting background investigations.

Over the next year, the Detachment continued to grow. By January 1945, it included 148 officers and 161 enlisted agents. This included non-CIC military personnel with specific technical abilities critical to the security of the program. Detachment Headquarters was centralized at Oak Ridge, but personnel were placed on detached service within 11 branch offices around the nation. At times, these agents were so highly classified that they were referred to by code symbols and only the Finance Officer computing the pay of the agent knew his exact location.

After the war, the functions of intelligence and security were separated. The Area Engineer assumed responsibility for all production security, shipment security, personnel clearances, security education and minor security violations. An Intelligence Unit had responsibility for conducting investigations of subversive allegations against personnel, sabotage, and safeguarding military information type violations.

The procedures put in place by Lansdale and his CIC Detachment led to the successful protection of the atomic bomb program, later called the "War's Best Kept Secret."

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