We’ll have more on this topic in future, based on our archive of declassified Pentagon and CIA reports on US infiltration of North Korea during and after the war (TLDR: The operations usually ended in failure, including the capture of a US Army intelligence officer who has never been returned or accounted for by Pyongyang. Penetrating North Korea, which constantly prepares to thwart such operations, is certainly one of the most difficult missions possible for US special operators.
The issue of US clandestine military operations against North Korea has come to the fore with a New York Times report on an ill-fated 2019 special operations mission on North Korean territory. “A group of Navy SEALs secretly entered North Korea in early 2019 to plant a device to intercept the communications of dictator Kim Jong Un — but ended up firing on unarmed civilians diving for shellfish, puncturing their lungs to conceal their bodies and aborting the mission, according to a New York Times report,” summarizes the NY Post (NYTimes story is paywalled.)
The bottom line: Infiltrating North Korea via the ocean (or land and air) is extraordinarily difficult. The country has built a surveillance state with strong emphasis on preventing such penetrations. Local villagers and fishermen are ordered to be watchful for enemy infiltrators and report them at once. Pyongyang also takes many other measures to protect its coast. In past, US intelligence has indicated that North Korea laid barriers of large fish hooks in areas where divers might attempt to land. In South Korea, soldiers raked the sand of vulnerable beaches to show the footprints of infiltrations — a tactic no doubt emulated in the North. Meantime, surveillance technology has dramatically improved, providing additional means to detect infiltrators. [It’s also not easy to infiltrate South Korea via the sea, which North Korea has done many times since the end of the war. In 1996 a North Korean special operations submarine ran aground off a South Korean beach, leading to the deaths of numerous North Korean operators (many killed by their own team) and a massive manhunt across South Korea.]
We’ve been researching for years US and South Korean operations against North Korea, and North Korean special operations against South Korea and US facilities. This includes substantial use of the Freedom of Information Act and research in the National Archives concerning the history of the Korean Demilitarized Zone (DMZ) and US/North Korean military engagements there and across the Korean area of operations, along with the fate of US POWs captured by the communists during the Korean War but never returned or accounted for. Many files from the mid-1950s onward remain classified, but there’s reason to believe the defenses America and South Korea had to penetrate in previous decades are even more substantial now.
Note: The author of this post brings a useful perspective on these issues. He led US operations against North Korean infiltrators in the Korean DMZ during the 1980s, was trained in special forces underwater operations and infiltration, and studied Korean history and language. He has also visited and conducted research in North Korea.
Failures of US Infiltrations of North Korea: Capture of US Army Officer on Mission
Previously classified files covering the 1950s demonstrate the challenges of US and South Korean covert operations in North Korea and other still-secret records no doubt hold more details. Some 10,000 South Korean agents infiltrated the North from the latter stages of the Korean War until 1972 (some accounts giving a later end date); about 8,000 never returned. This is based on RoK government statements and testimonials of South Korean agents who managed to return alive from North Korea.
Our research has also provided details on a US Army intelligence officer captured during a sea-borne infiltration of North Korea near the end of the Korean War.
CIA Files on Maritime Operations Against North Korea
The difficulty of martime operations against North Korea was proven during the Korean War. Declassified analysis of joint CIA and US military operations to insert and resupply agents in North Korea during and after the war found them generally unsuccessful, leading to the deaths of significant numbers of Korean agents working for the US, plus the capture of US Army intelligence officer Leonard Button (see below.)
The bottom line: Once the communist regime consolidated its control of North Korea during the latter years of the war and after, penetration missions became increasingly difficult. Inserting agents was highly dangerous and they were usually killed, or captured and doubled, not long after the landings. Aerial missions had similar problems and land infiltration was generally effective only within a few miles of the Demilitarized Zone/front lines.
Excerpt from “CIA in Korea, 1946-65 — Vol. 1”
Months after Pyongyang’s 1950 invasion of the South (with substantial help from the Soviets and communist Chinese), US and South Korean forces conducted a bold amphibious operation. The Inchon Landing was designed to cut off communist forces. Seaborne intelligence and commando operations behind enemy lines helped make the landing a success. But after the front lines stabilized later in the war, operations against North Korean-controlled territory became increasingly difficult. In the second year of the war, American maritime penetrations of North Korea supported by the Navy’s Underwater Demolition Team/UDT, precursor unit to the SEALS, proved successful for some limited objectives, according to a partially declassified CIA history.
But most other infiltration operations, often involving insertions of South Korean agents trained by the US, failed. A CIA history of operations against North Korea from 1946 to 1965, partially declassified, concludes that agent infiltrations, many maritime, during and right after the war, were a failure.
Some CIA Info That Remains Unavailable
Documents declassified in recent years (including the one below from an ongoing lawsuit by POW/MIA family members and the publisher of this site), reveal post-war agent insertions in North Korea by US Army intelligence. The operations became increasingly difficult in the summer of 1954 due to increased North Korean naval patrols: “This added security provided further deterrents to agent operations in this area.” The documents also reveal that RoK intelligence, and even in one case US Army counterintelligence personnel, impeded Army intelligence efforts to recruit and train South Korean civilians for operations in the North and “stay-behind” missions in South Korea, for use if North Korea overran part of the country.