Two sonic booms told the American prisoners in Vietnam's infamous Hoa Lo prison – the Hanoi Hilton – their escape plan was a 'go.'
On May 2 and 4, 1972, two SR-71 Blackbirds overflew Hanoi, North Vietnam at noon. The first plane broke the sound barrier, causing an ear-splitting sonic boom over the city. Fifteen seconds later, the other Blackbird did the same thing.
Prisoners at Hoa Lo developed a code-tapping language to communicate with each other. Capt. James Stockdale, who was the senior ranking officer at the prison, taught many incoming POWs this code. It kept the men sane and their spirits up.
Communicating with Washington was trickier. Three months into his captivity, Stockdale was allowed to write to his wife, Sybil. Two months later, he was allowed to write again. When she received the letters, she found them confusing. Nicknames and references to their mutual friends were wrong.
Sybil gave the letters to Naval Intelligence in San Diego who figured out he was using doublespeak – deliberately misleading language –to let his superiors know he was not being treated well in North Vietnam. With her cooperation, the CIA and Office of Naval Intelligence decided to use her correspondence back to her husband as a way to communicate with the prisoners.
Her first letter included a Polaroid of her with a secret message sandwiched between the sheets of photographic paper. It explained the process of using invisible ink to send messages to the CIA. He listed the other POWs with him and detailed the abuses inflicted on American prisoners there.
The new communication policy allowed the prisoners and the CIA to trade a wealth of information, so much so that the prisoners were actually able to assemble a small shortwave radio, which was eventually discovered during an inspection).
In 1969, two prisoners, Air Force Captains John Dramesi and Edwin Atterberry, escaped from the prison at Cu Loc but were recaptured the next day. Massive reprisals from their captors followed, and thus the prisoners' leadership determined the retribution was too much and escape attempts should only be made with a "high likelihood of success and assurance of outside assistance." That's when they came up with the Red River plan.
Members of the escaping POW group sent their plan to the U.S. Defense Secretary Melvin Laird approved the plan in January 1972. By May, everything was in place. The sonic booms were a go.
Despite a few setbacks, members of SEAL Team One and Underwater Demolition Team Eleven used SEAL Delivery Vehicles (SDV – mini-submarines) and HH-3A helicopters to patrol the coastline throughout May and June looking for escaped POWs. They never found any.
As the senior ranking officer, Stockdale forbid any escape attempts. He judged the plan too risky and the threat of reprisals too harsh. (Prisoners were often killed during these reprisals). The would-be escapees were frustrated by the policy, but they obeyed.
Article III of the Code of Conduct for prisoners does say American POWs should make every effort to escape captivity. Article IV, however, prohibits any action that would cause harm to other captured personnel. So Thunderhead was terminated.
The POWs would communicate with Washington throughout the war. Eventually, another radio was smuggled in, which gave POWs a direct line from the camp to the U.S. Seventh Fleet commanders aboard ships in the Gulf of Tonkin.
In January 1973, 591 POWs were repatriated back to the United States. For his leadership among the prisoners and work to galvanize the resistance to their captors, Stockdale received the Medal of Honor from President Gerald Ford.