The plane crashed 90 nautical miles (167 km) off the North Korean coast and all 31 Americans (30 sailors and 1 Marine) on board were killed, which constitutes the largest single loss of U.S. aircrew during the Cold War era.

[2] The Nixon administration did not retaliate against North Korea apart from staging a naval demonstration in the Sea of Japan a few days later, which was quickly removed.

Nine of the crew, including one Marine NCO, were Naval Security Group cryptologic technicians (CTs) and linguists in Russian and Korean.

[2] Its flight profile involved taking off from NAS Atsugi then flying northwest over the Sea of Japan until it came to an area off Musu Point, where the EC-121M would turn northeast toward the Soviet Union and orbit along a 120-nautical-mile (222 km) long elliptical track similar to that of a racetrack; once the mission was complete, they were to return to Osan Air Base, South Korea.

[5] LCDR Overstreet's orders included a prohibition from approaching closer than 50 nautical miles (90 km) to the North Korean coast.

During the first three months of 1969 nearly 200 similar missions had been flown by both Navy and U.S. Air Force reconnaissance aircraft off North Korea's east coast without incident.

[3] The communications that this activity generated within the National Security network were monitored by the EC-121's parent unit, VQ-1, which sent Deep Sea 129 a "Condition 3" alert by radio at 13:44, indicating it might be under attack.

Referring to the EC-121 as the "plane of the insolent U.S. imperialist aggressor army,"[6] the North Korean media accused it of "reconnoitering after intruding deep into the territorial air.

Initially, the Task Force comprised the aircraft carriers Enterprise, Ticonderoga, Ranger, and Hornet with a screen of cruisers and destroyers that also included the battleship New Jersey.

[6] On April 16, the United States National Security Council considered the following options:[5] General Earle Wheeler, chairman of the Joint Chiefs of Staff, presented to the NSC a plan to attack North Korea with missiles carrying atomic warheads of 30 kilotons each, roughly equal to two-and-half times the bomb dropped on Hiroshima.

A day later, National Security Adviser Henry Kissinger told Nixon by telephone that, given the prospect of retaliation with the use of strategic warheads, "We might have to go to tactical nuclears.

[5] Other sources claim that this attack may have taken place to honor Kim Il Sung, as his birthday fell on April 15, but not enough evidence exists to support this statement.