On January 23, 1968, the Navy's U.S.S. Pueblo was coasting in the waters off the Democratic People's Republic of Korea. The boat had been sent by Rear Admiral Frank L. Johnson and its mission, approved by then-President Johnson's National Security Council, was to intercept communications from communist North Korea. As part of Cold War reconnaissance, the Navy and the National Security Agency wanted an update on North Korea's military, and the U.S.S. Pueblo—a specialized spy ship packed with advanced sensors and sensitive encryption devices—was the tool for the job.

For weeks the Pueblo sat, monitoring Korea's communications. On January 21, the ship was buzzed by a submarine chaser. The next day, a pair of fishing trawlers made an aggressive approach, but they, too, left without incident. A day later, say John Prados and Jack Cheevers, writing for the National Security Archive, the Korean navy showed up in force:

Pueblo was not again approached until around mid-day on January 23, when a North Korean submarine chaser followed by three torpedo boats closed on her and ordered Bucher's ship to heave to. The American skipper turned his vessel toward the open sea but the Pueblo, a slow ship, had no chance of outrunning her pursuers and the North Korean warships opened fire with cannon and machineguns. The Pueblo was captured, taken to Wonsan, and Commander Bucher and his crewmen began 335 days in captivity.

One crew member was killed in the attack, the other 82 “were threatened with death, interrogated, and some were severely beaten.” But on top of the loss of the Pueblo and her crew was the falling of the NSA's encryption equipment into North Korea's hands.

The attack on the Pueblo, and the major threat it posed to American intelligence security, say Cheevers and Prados, sparked a flurry of brainstorming in the Johnson administration as to how the U.S. should respond. Through a series of recently declassified documents, dug up by Cheevers in preparation for his book, Act of War: Lyndon Johnson, North Korea, and the Capture of the Spy Ship Pueblo, Cheevers and Prados reconstruct the retaliations nearly doled out on the People's Republic of Korea.

The Johnson administration considered several risky courses of action to retaliate for the Pueblo seizure. They included a blockade of North Korean ports, air strikes on military targets, an attack across the Demilitarized Zone separating the two Koreas, a phony intelligence leak to the Soviets that the United States planned to attack North Korea, and a "show of force" by U.S. naval and air units outside the port of Wonsan, where the Pueblo was being held.

President Johnson shot down these aggressive response plans, deciding instead that diplomacy was the best bet to get the Pueblo's crew home safe. But, says Ohio State University's Mitchell Lerner, Johnson did prepare a number of contingency plans.

One of these plans, detailed in a May 1968 document, revealed one particularly striking idea. Cheevers and Prados:

In the tense aftermath of the Pueblo seizure, Pentagon war planners weighed using nuclear weapons to stop a possible communist invasion of South Korea, as well as mounting a massive air attack to wipe out North Korea's air force. The nuclear option, eerily codenamed "Freedom Drop," envisioned the use of American aircraft and land-based missiles to incinerate onrushing North Korean troops.

“After 11 months of torture and starvation,” says USA Today, “the crew of the Pueblo was released in December 1968 after a series of negotiations with the North Koreans and a false apology by the United States. The crew returned home to a nation weary of war but overjoyed by their safe homecoming.”

The USS Pueblo, however, stayed in North Korea. Over the summer, the ship was trotted out as part of a ceremony in Pyongyang—it's been painted, says UPI, and is now part of a war museum.