The Korean DMZ Conflict, also known as the Second Korean War, was a series of armed clashes between North Korea and South Korea, supported by the United States, along the Korean Demilitarized Zone (DMZ) between 1966 and 1969.

Background The Korean War had devastated both North and South Korea, and while neither side renounced its claims to reunify Korea under its control, neither side was in a position to force reunification.

In September 1956, Chairman of the Joint Chiefs of Staff Admiral Radford indicated within the U.S. government that the military's intention was to introduce atomic weapons into Korea, which was agreed to by the United States National Security Council and President Eisenhower. However paragraph 13(d) of the Korean Armistice Agreement mandated that both sides could not introduce new types of weapons into Korea, thus preventing the introduction of nuclear weapons and missiles. The U.S. decided to unilaterally abrogate paragraph 13(d), breaking the Armistice Agreement, despite concerns by United Nations allies. At a 21 June 1957 meeting of the Military Armistice Commission, the U.S. informed the North Korean representatives that the U.N. Command no longer considered itself bound by paragraph 13(d) of the armistice. In January 1958, nuclear-armed Honest John missiles and 280mm atomic cannons were deployed to South Korea, followed within a year by atomic demolition munitions and nuclear-armed Matador cruise missiles capable of reaching China and the Soviet Union.

North Korea denounced the abrogation of paragraph 13(d) as an attempt to wreck the armistice agreement and turn Korea into a U.S. atomic warfare zone. It responded by digging massive underground fortifications resistant to nuclear attack, and forward deployment of its conventional forces so that the use of nuclear weapons against it would endanger South Korean and U.S. forces as well. In 1963, North Korea asked the Soviet Union for help in developing nuclear weapons, but the Soviets refused. China later, after its own nuclear tests, also rejected North Korean requests for help.

In North Korea, the departure of the Chinese People's Liberation Army in October 1958 allowed Kim Il Sung to consolidate his power base and embark on the Chollima Movement of collectivised agriculture and industrialization to build a base for reunifying Korea by force. North Korea remained dependent on the Soviet Union for technology and on China for agricultural assistance.

Following the war, South Korea remained one of the poorest countries in the world for over a decade. In 1960, its gross domestic product per capita was $79, lower than most Latin American and some sub-Saharan African countries. The April Revolution that forced President Syngman Rhee from office in April 1960 was followed by a brief period of democracy before a coup d'état led to General Park Chung Hee seizing power in May 1961. Despite the political turmoil, the South Korean economy continued to grow, led by the industrial sector. Rapid industrial growth started in the late 1960s, with gross domestic product per capita rising from $100 in 1964 to $1000 in 1977.

On 10 December 1962, Kim proposed a new military strategy to the Central Committee of the Workers' Party of Korea, with an increased emphasis on irregular warfare, agitation and propaganda, to be achieved by the end of the current Seven-Year Plan in 1967.

In June 1965, President Park signed a treaty normalizing relations with Japan, which included payment of reparations and the making of soft loans from Japan and led to increased trade and investment between the two countries. In July 1966, South Korea and the United States signed a Status of Forces Agreement, establishing a more equal relationship between the two nations. With its growing economic strength and the security guarantee of the United States, the threat of a conventional invasion from the north seemed increasingly remote. Following the escalation of the Vietnam War with the deployment of ground combat troops in March 1965, South Korea sent the Capital Division and the 2nd Marine Brigade to South Vietnam in September 1965, followed by the White Horse Division in September 1966.

The start of the DMZ Conflict can be traced to a speech given by North Korean leader Kim Il Sung on 5 October 1966, at the Workers' Party of Korea Conference, where the status quo of the 1953 Armistice Agreement was challenged. He apparently perceived that the division of effort by the South Korean military and the ever-growing escalation of the US commitment in Vietnam had created an environment where irregular warfare could succeed in a way conventional warfare could not. Kim believed that he could force a split between the U.S. and South Korea through armed provocations targeting U.S. forces that, together with other worldwide commitments and small wars, would force the U.S. to reassess or relinquish its commitment to South Korea, allowing North Korea to incite an insurgency in the South that would topple the Park administration.

There were ongoing propaganda campaigns between the north and south, such as loudspeaker broadcasts across the DMZ at each other. Leafleting of North Korea had been resumed, such as Operation Jilli from 1964 to 1968 which delivered a few hundred million leaflets to the north.

Military forces

North Korea In 1966, the Korean People's Army (KPA) deployed eight infantry divisions along the DMZ, backed by eight more infantry divisions, three motorized infantry divisions, a tank division and a collection of separate infantry and tank brigades and regiments. Its total armed forced personnel numbered 386,000. While strong, this conventional force was smaller than the South's ground forces of about 585,000, and it was unlikely that the North could deliver a knockout blow before the U.S. could deploy additional forces.

The main unconventional warfare arm was the Reconnaissance Bureau of the Ministry of Defense under the operational control of the Liaison Department of the Workers' Party of Korea which included the 17th Foot Reconnaissance Brigade and the all-officer Unit 124 and 283rd Army Units. These units were all highly trained and indoctrinated, skilled in demolitions and small-unit tactics, they would usually operate in small teams of 2–12 men, lightly armed with either PPS submachine guns or AK-47s. The Reconnaissance Bureau also controlled the 23d Amphibious Brigade, which used specially made infiltration boats to operate along the South Korean coastline. The Reconnaissance Bureau could also use conventional KPA and Korean People's Navy forces to support the infiltration and exfiltration of its teams.

In addition to the offensive irregular forces, North Korea also deployed several thousand operator-agitators to select, train, and supervise informants and guerrilla recruits, while others attempted to cause individual defections and unit dissatisfaction in the ROK military and generally undermine the morale of both the ROKs and the Americans.

United States and South Korea The major U.S. ground combat units in Korea were the 2nd Infantry Division (2ID) and 7th Infantry Division (7ID), I Corps and 8th Army. 2ID stood with the 3rd Brigade manning 29.8 km of the Korean Demilitarized Zone (DMZ) essentially due north of Seoul on either side of Panmunjom, with another nine Republic of Korea Army (ROK) Divisions manning the remaining 212.8 km of the DMZ. All U.S. and ROKA forces were under the unified operational control of United Nations Command (Korea) (who was also the commander United States Forces Korea), General Charles H. Bonesteel III. Both U.S. Army Divisions were seriously under strength as Vietnam had priority for manpower and equipment. Troops were equipped with M14 rifles rather than M16s, the only available tanks were older gasoline powered M48A2Cs, and there were a total of only 12 UH-1 Huey helicopters in South Korea seriously restricting the ability to hunt and engage infiltrators. Troops were generally draftees serving a 13-month tour, while experienced officers and NCOs preferred service in Vietnam to Korea. The ROK Divisions were well trained and highly motivated with many of the officers and NCOs veterans of the Korean War, but all their equipment dated back to that war; their standard rifle was still the M1 Garand.

The main operational objective of the US and ROK Divisions was to defend against a conventional invasion from North Korea in a repeat of the attack of June 1950. While there were regular infiltrations into the South for intelligence-gathering, unconventional warfare was not seen as a particular threat and the troops were not generally trained or equipped for this role. No counter-guerilla units or village militias existed in South Korea in 1966 and infiltrators were variously hunted by the ROK Army, Police and the Korean Central Intelligence Agency with no unified control.

In 1976, in now-declassified meeting minutes, U.S. Deputy Secretary of Defense William Clements told Henry Kissinger that there had been 200 raids or incursions into North Korea from the south, though not by the U.S. military. Details of only a few of these incursions have become public, including raids by South Korean forces in 1967 that had sabotaged about 50 North Korean facilities. Up to 7,700 South Korean soldiers and agents infiltrated North Korea from the end of the Korean war until 1972, an estimated 5,300 of whom never made it back.

The ground forces were supported by fighter-bombers of the USAF 314th Air Division and by the Republic of Korea Air Force. The seas around Korea were under the control of the United States Seventh Fleet and the Republic of Korea Navy. As with the Army, the war in Vietnam was the main focus of the USAF and the USN in the Pacific.

Defense strategies From October 1964 the North Koreans increased the infiltration of intelligence-gatherers and propagandists into the South. By October 1966, more than 30 South Korean soldiers and at least 10 civilians had been killed in clashes with North Korean infiltrators; however, no similar clashes had taken place along the U.S.-controlled section of the DMZ. In October 1966, South Korea staged a retaliatory attack without seeking the approval of General Bonesteel, causing tension between the U.S. command, which wished to avoid violations of the armistice, and the South Koreans, who were suffering ongoing losses.

Following the first KPA attack on US forces in November 1966, Bonesteel formed a working group to analyse the North Korean strategy and develop a counter-strategy to defeat it. Finding the existing U.S. Army tactical doctrines to be inapplicable to the situation they were facing, the working group developed its own doctrine to meet its operational needs. Three types of operations were identified: first to guard against infiltration across the DMZ; second was a similar naval effort along the coasts; and third was counterguerrilla operations in the interior. All three types of operations had to be accomplished without jeopardizing the conventional defense of South Korea or escalating the conflict from low-intensity to a full war.

Demilitarized Zone