6. The United States counterintelligence effort so secret the President wasn’t aware of it
In June 1942, the United States Army took over a disused girl’s school building known as Arlington Hall, just outside Washington DC. There the Army’s Signals Intelligence Service established code-breaking and signals monitoring activities, which during the war focused on the codes used by Japan. A Russian Section, created during the war, concentrated on activities in the Pacific as well. The activities of the Signals Intelligence Service remained one of the most closely guarded American secrets of the war. Neither President Franklin Roosevelt nor his successor Harry Truman were made aware of the work at Arlington Hall during and after the war. It later merged with other agencies to form the NSA.
The President didn’t know of it, but by 1945 the Soviets did, infiltrating Arlington Hall that same year with a Ukrainian-American NKVD agent. The following year an analyst at the facility managed to break the Soviet spy agencies‘ codes to the extent they could read messages created during the war years. One such message, written in 1944, contained the names of several scientists who had worked on the Manhattan Project. The Soviet signal traffic was assigned to the Venona project for analysis. Created in February 1943, the Venona Project 1946 revealed the espionage activities surrounding the Manhattan Project. It also revealed the existence of Soviet espionage in Washington in the State Department, the Department of the Treasury, and in several foreign embassies and legations in the United States.