Екінші дүниежүзілік соғыста АҚШ Әскери-теңіз күштерінің сигналды барлау бөлімі

WWII US Navy signals intelligence group

OP 20 G or "Office of Chief Of Naval Operations (OPNAV), 20th Division of the Office of Naval Communications, G Section / Communications Security", was the U. S. Navy's signals intelligence and cryptanalysis group during World War II. Its mission was to intercept, decrypt, and analyze naval communications from Japanese, German, and Italian navies. In addition OP 20 G also copied diplomatic messages of many foreign governments. The majority of the section's effort was directed towards Japan and included breaking the early Japanese "Blue" book fleet code. This was made possible by intercept and High Frequency Direction Finder (HFDF) sites in the Pacific, Atlantic, and continental U. S., as well as a Japanese telegraphic code school for radio operators in Washington, D. C.

In the early hours of the morning of 7 December 1941, the U. S. Navy communications intercept station at Fort Ward on Bainbridge Island, Washington, picked up a radio message being sent by the Japanese government to the Japanese embassy in Washington, D. C. It was the last in a series of 14 messages that had been sent over the previous 18 hours. The messages were decrypted by a PURPLE analogue machine at OP 20 G and passed to the SIS for translation from Japanese, early on the morning of December 7. Army Colonel Rufus S. Bratton and Navy Lieutenant Commander Alwin Kramer independently inspected the decrypts. The decrypts instructed the Japanese ambassador to Washington to inform the US Secretary of State, Cordell Hull, at 1:00 PM Washington time that negotiations between the United States and Japan were ended. The embassy was then to destroy their cipher machines. This sounded like war, and although the message said nothing about any specific military action, Kramer also realized that the sun would be rising over the expanses of the central and western Pacific by that time. The two men both tried to get in touch with Army Chief of Staff General George C. Marshall. After some agonizing delays, Marshall got the decrypts and methodically examined them. He realized their importance and sent a warning to field commanders, including Major General Walter Short, the Army commander in Hawaii. However, Marshall was reluctant to use the telephone because he knew that telephone scramblers weren't very secure and sent it by less direct channels. Due to various constraints and bumblings, Short got the message many hours after the Japanese bombs had smashed the US Navy's fleet at anchor in Pearl Harbor.

In Summer 1942 the Navy went through the motions of perhaps co locating with the Army's SIS but Commander Joseph Wenger had picked out the "perfect new home" for the rapidly expanding OP 20 G and commandeered a private girls' school Mount Vernon College for Women for $800,000 (a fraction of what the buildings and grounds were worth), in 1944, compensated $1.038 million. So on 7 February 1943 it opened at what was called the "Naval Communications Annex", and staff moved in over the next two months.