South Korea’s Revitalized “Three-Axis” System

As North Korea continues to advance its nuclear and missile capabilities, posing a significant threat to South Korea’s national security, South Koreans are calling for a swift and strategic response. One of the Yoon Suk-yeol administration’s main response measures is the so-called “Three-Axis” system. The Three-Axis system, designed after North Korea’s fifth nuclear test in 2016, is South Korea’s military strategy to counter North Korea’s growing nuclear and missile threats. The Ministry of National Defense (MND) announced the establishment of a “strategic command” by 2024 to manage the Three-Axis system and expand response options against North Korea’s nuclear and ballistic missiles. The new strategic command will utilize key military assets, including F-35As, reconnaissance satellites, missile interceptors, and ballistic missiles, from all three branches of the South Korean military.

As a military system designed to track, detect, and eliminate an adversary’s ballistic missiles and weapons of mass destruction (WMDs), South Korea’s Three-Axis system aims to defend its territory by developing a Kill Chain to preemptively strike the source of an attack, intercepting the incoming missile strikes using the Korea Air and Missile Defense (KAMD), and employing the Korea Massive Punishment and Retaliation (KMPR) campaign to eliminate the adversary’s command-and-control by neutralizing its leadership and military facilities.

Kill Chain

The first axis is a “Kill Chain,” a preemptive strike against the adversary’s ballistic missiles or WMDs. If there are signs of an imminent attack by North Korea to launch a ballistic or nuclear-loaded missile targeting South Korea, Kill Chain aims to preemptively eliminate the threat by initiating a precision strike against North Korea’s military assets, such as missile silos, before the missile is launched.

Kill Chain was announced in 2013, shortly before North Korea’s third nuclear test. The MND projects the full deployment of weapons systems and military assets for Kill Chain by the mid-2020s and intends to carry out detection, identification, decision, and strike in less than thirty minutes to maximize the effectiveness of Kill Chain.

The problem, however, is that South Korea lacks the reconnaissance satellites to detect and identify North Korea’s missile attack. Due to the missile restriction guideline imposed by the United States in 1979, South Korea was prohibited from developing rockets capable of conducting geospatial intelligence activities. This has forced the country to rely on U.S. intelligence assets when monitoring North Korea and its military movements. The current aerial reconnaissance over North Korea is mainly conducted by U.S. reconnaissance planes like RC-135W Rivet Joint and E-8C. In June 2021, the U.S. government lifted the missile guideline imposed on South Korea, allowing South Korea to expand its missile and space force capabilities. In response, the South Korean government plans to launch five surveillance satellites in partnership with Space X by 2025, with the first launch planned for the end of 2023.

Lately, North Korea’s concern about the preemptive nature of Kill Chain has played a role in the escalation of inter-Korean tensions. In April 2022, Kim Yo-jong implied that South Korea’s provocative military actions, such as enhancing its preemptive strike capabilities, would create a more conducive environment for North Korea to use nuclear combat force and escalate tensions on the Korean Peninsula. Pyongyang has continuously displayed a high level of hostility against Seoul’s indication of a preemptive strike as it lacks the missile defense capability to detect and intercept incoming missiles. Kim Yo-jong’s statement was made soon after South Korean Defense Minister Suh Wook affirmed the readiness to launch precision strikes against North Korea if North Korea’s missile launch signals were to be detected. Since then, coinciding with President Yoon’s inauguration, North Korea’s missile tests have diversified. It began to launch multiple missiles from different sites, intending to complicate South Korea’s early warning and preemptive strike capability.

A major question surrounding the development and implementation of Kill Chain is whether the system falls under the lawful use of force in the name of anticipatory self-defense. Military experts generally claim Kill Chain as a lawful use of force but with several conditions. First, there should be clear and irrefutable evidence that a North Korean first strike is imminent. This includes early detection via surveillance assets of preparations for a ballistic missile or WMD attack that targets South Korean soil. Second, a preemptive strike must satisfy the principle of proportionality as laid out under international law, which becomes a dilemma for South Korea as a country cannot measure the proportionality of its response without first suffering the damage of an attack. South Korea could estimate the potential magnitude of harm that a North Korean missile strike would inflict based on the payload and type of missiles. However, with a limited response time, measuring the extent of potential damage and proportionally calibrating a preemptive strike would be difficult. For South Korea to make Kill Chain justifiable and effective, it needs to carefully review the proportionality of and refine the preemption strategy under multiple scenarios of North Korea’s attack.

Korea Air and Missile Defense (KAMD)

The second axis is “Korea Air and Missile Defense,” known as KAMD, which intercepts launched missiles using a multi-layered missile defense system. If Kill Chain fails to preemptively strike the North Korean missiles, early warning radar aims to detect the incoming missiles, allowing missile interceptors such as the guided-missile destroyer and surface-to-air missiles to eliminate them before they strike.

KAMD is a multi-layered system with different types of missiles launched at varying altitudes. Patriot-2 and Patriot-3 (medium-range surface-to-air missiles or M-SAM) engage at an altitude of twenty kilometers and below, while Cheongung (M-SAM) intercepts missiles flying at an altitude of twenty to forty kilometers. These missile systems are designed to target approaching missiles at the terminal phase. Additionally, the Terminal High Altitude Area Defense (THAAD) is deployed at altitudes above forty kilometers, defending against missiles at higher altitudes that Patriots and Cheongung do not cover.

Since KAMD is defensive, it is mostly free from international legal constraints. However, from a technical point of view, there are significant loopholes in its defense against North Korea’s sophisticated ballistic missiles. Hwasong-8, North Korea’s hypersonic missile, is one of the major threats that KAMD faces. The missile’s hypersonic speed, low flight altitude, and terminal-phase maneuverability may cause difficulties for the South Korean missile defense system in calculating the trajectory and speed of an incoming missile. With South Korea’s recent missile test failure, the credibility of KAMD is subject to even greater doubt.

With North Korea’s advancing ballistic missile capability, South Korea’s missile defense system is becoming more vulnerable. There is an urgent need for the South Korean government to strengthen its missile defense system by investing in and adding advanced missile interceptors, such as boost or midcourse phase interceptors, that can detect and neutralize missiles faster and defend against advanced weapons like Hwasong-8. South Korea should also expand the U.S.-South Korea-Japan trilateral coordination. Facing increasing North Korea’s nuclear and missile threats, the three countries can ramp up collective efforts via joint military exercises to enhance the detection, tracking, and interception of North Korea’s missiles.

Korea Massive Punishment and Retaliation (KMPR)

The third axis, “Korea Massive Punishment and Retaliation” (KMPR), involves striking Pyongyang in retaliation for North Korea’s nuclear or conventional first strike on South Korea. It targets the North Korean leadership, including Kim Jong-un, by targeting his potential hideouts and military bunkers, and destroys the North’s command-and-control structure, premised on deterrence-by-punishment.

Under KMPR, the multiple rocket launcher (K239 Cheonmu), surface-to-surface missile (ATACMS), bunker buster (GBU-28), and air-to-surface missile (AGM-84H/K SLAM-ER) are utilized. K239 Cheonmu and ATACMS can strike targets 300 km away, while GBU-28 can penetrate the enemy’s military bunkers (up to 6 m of concrete). AGM-84H/K SLAM-ER operates as a long-range air-launched precision strike against targets on land and sea, flying 280 km at maximum. By specifying the North Korean leadership as the primary target of an attack, KMPR aims to reinforce deterrence against North Korea, signaling to the North that its conventional or nuclear attack will directly jeopardize its regime’s existence.

Among the three axes, KMPR faces the most controversy due to the sensitivities it evokes from North Korea and its potentially disproportionate nature. As the term entails, KMPR is an intentional and heavy retaliation targeting North Korean leadership. However, it fails to fulfill the condition of proportionality that international law requires. The Customary International Humanitarian Law (Rule 145. Reprisals) limits the state’s reprisal to target only combatants and military objects. Targeting the North Korean leadership, including the supreme leader and party officials who do not bear arms, would be disproportionate, violating international humanitarian law depending on the scope and impact of North Korea’s initial actions. Military retaliation must be proportionate to the violation it aims to stop. Thus, KMPR’s classification of the North Korean leadership as the eliminatory target does not meet this requirement.

Another concern is the risk of North Korea’s subsequent retaliation, in which the situation is likely to escalate further. Assuming North Korea’s initial engagement as a conventional missile strike, South Korea’s KMPR could shift the conflict from a conventional to a nuclear level. Facing the South’s retaliatory strike against its leadership, the North would be more inclined to pursue a heavier attack on South Korea, potentially bringing its nuclear weapons into use. This will disadvantage South Korea since it does not possess nuclear weapons. One should also recognize that North Korea’s nuclear weapons have attained qualitative and quantitative advancement, possibly allowing the North to achieve a survivable nuclear force. KMPR would likely trigger North Korea’s heavier tit-for-tat retaliation, pushing it into a “use-it-or-lose-it” dilemma, which would inflict significant damage to South Korea and spark a nuclear escalation in the Korean Peninsula.

Conclusion

As North Korea continues to advance its nuclear and missile capabilities, explicitly targeting South Korea, the South Korean government should swiftly complete its revitalization of the Three-Axis system and deploy additional military assets to fill the gap and vulnerabilities mentioned above. More advanced and sophisticated surveillance and reconnaissance systems must be introduced to bolster Kill Chain. The government should conduct a full-scale check and modification of KAMD, reinforcing early detection and adding boost and midcourse phase interceptors to raise its defensive capability against North Korea’s advancing ballistic missile program. Last but not least, the legality of KMPR should be proactively discussed at the ministerial level in South Korea. The core elements of KMPR, namely the targeting of North Korean leadership, should be reframed as a lawful and legitimate use of force under international law.