In 1939, when the Soviet Union was an ally of nazi Germany, the u.S. Army began collecting copies of encrypted cables sent commercially to Moscow by the Soviet diplomatic missions in the U.S. No effort to decrypt the cables, thought to be diplomatic in nature, was made until 1943, when reports were recieved that Stalin, by then an ally of the United States, was negotiating a seperate peace treaty with Germany. At that time, the Army Signals Security Agency (SSA), an early predecessor of the National Security Agency (NSA), was oredered to establish a program eventually called Venona to decipher the cables.
Peake, Hayden B.
"The Venona Progeny,"
Naval War College Review: Vol. 53
, Article 10.
Available at: http://digital-commons.usnwc.edu/nwc-review/vol53/iss3/10