Piercing the Fog - Air Force Historical Studies Office

Piercing the Fog
An Enigma machine captured
from the German Army in
France in August 1944.
communications unit (SCU), which operated the radio equipment for receiving
and sending signals, and a special liaison unit (SLU) responsible for deciphering,
physically controlling, and destroying ULTRA material. SCU and SLU
members served with every senior field commander who received ULTRA
information. Dissemination of Enigma-generated intelligence-and especially
its source-was tightly restricted, particularly in the early years, to a very small
number of senior commanders and staff officers. Accordingly, a new security
classification designator was created: TOP SECRET ULTRA. Over time, the
designation ULTRA came to be applied to that intelligence derived from the
German Enigma machine traffic and to decrypted Japanese traffic as well.
Early in 1941, a small delegation of experts working for the U.S. Army's
Signal Intelligence Service went to London for conferences with the British on
cryptographic technology. The meetings were an outgrowth of the October 1940
visit to the United States by Sir Henry Tizard, one of the guiding mentors of the
British radar defense system, and several others. The Tizard Mission brought
to America a number of new scientific-military developments including the
cavity magnetron, essential to generating microwave radar signals. Following
the American visit to London, the British received copies of the American
PURPLE and RED machines for use in decrypting Japanese diplomatic and naval
radio traffic. In return, although after some delay, the British sent one of their
copies of the German Enigma machines to Wa~hingt0n.l~
American involvement in the operational aspects of Great Britain's ULTRA
system evolved only slowly after December 1941. This growth was in marked
contrast to the previously concluded extensive Anglo-American exchanges of
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