On 7 February 2016, South Korea announced consultations over the deployment of the Terminal High Altitude Area Defense (THAAD) system, which was officially announced to be implemented on 8 July 2016. The deployment was strongly opposed by China, North Korea and Russia. South Korea argued that this military deployment was only a defensive measure to deal with North Korea, while China believed that the deployment of the THAAD system would affect its national security and took a number of countermeasures against South Korea, including implementing a Hallyu ban. These measures also led to a rapid deterioration of relations between China and South Korea.

On 6 March 2017, the system's launchers arrived in South Korea, and the South Korean side said that the system would be in service as early as April. On 26 April, two launchers were deployed in Seongju County, North Gyeongsang Province, and on 12 September, the temporary deployment of the remaining four launchers was completed. All six launchers were confirmed to have normal fire control radar and were in operational status. On 22 November, China and South Korea reached a consensus, and the South Korean side stated that it "will not consider adding THAAD system, will not join the US anti-missile system, will not develop the trilateral military alliance of South Korea, the United States and Japan, and has no intention of harming China's security interests". Since then, relations between the two countries have gradually warmed up.

Background The development of the North Korean nuclear program was a major catalyst for South Korea's deployment of the Terminal High Altitude Area Defense (THAAD). Between 2009 and 2014, North Korea conducted two nuclear tests (2009 and 2013) and three long-range missile tests. A study by United States experts in February 2015 estimated that North Korea had 10–16 nuclear weapons in its stockpile and also estimated that North Korea’s fissile material production infrastructure had been modernized and expanded. In April 2015, Chinese experts said that North Korea may have 20 nuclear warheads and that it had enough uranium to increase to 40 the following year. In addition to developing weapons, North Korea made several statements targeting South Korea and the United States, including threats of a "pre-emptive nuclear strike of justice" and turning Washington, D.C. into a "sea of fire". In 2010, North Korea shelled Yeonpyeongdo in South Korea. The international community repeatedly attempted to resolve the issue of North Korea's program through talks, including the six-party talks. The United Nations Security Council passed resolutions prohibiting North Korea from testing any missiles or nuclear technology, but this failed to prevent North Korea's program from continuing to develop. From 2006 to 2016, the UN Security Council had passed three resolutions imposing sanctions on North Korea, including UN Security Council Resolution 1874. These multifaceted measures failed to achieve results. In December 2014, North Korea stated in a statement that "Now that the U.S. hostile policy with an aim to invade our republic under the guise of human rights has become apparent, the idea of denuclearizing the Korean Peninsula itself is no longer valid." On 16 March 2017, U.S. Secretary of State Rex Tillerson said during his visit to Japan that "The diplomatic and other efforts of the past 20 years to bring North Korea to a point of denuclearization have failed" and that a "different approach" is needed.

During this time, South Korea's main anti-missile weapon system was the deployment of the PAC-3 Patriot missile system. At the end of 2015, it deployed the M-SAM medium-range anti-missile system. However, it lacked high-altitude terminal defense equipment when dealing with large-scale ballistic missile attacks. The United States proposed to deploy the THAAD system on the Korean Peninsula in 2014, but South Korea has stated that it has not been asked by the US government to consult. However, China has repeatedly expressed its opposition and put pressure on South Korea. At a press conference on 28 May 2014, the Chinese Ministry of Foreign Affairs stated that "it will never allow tensions to arise at China's doorstep, leading to war and chaos", and also said that the deployment of the anti-missile system is "not conducive to regional stability and strategic balance". On 16 November 2014, China first mentioned that the deployment of THAAD would damage China-South Korea relations. Ambassador Qiu Guohong to South Korea said, "We firmly oppose the possibility of the THAAD system being deployed in South Korea. This will seriously damage China–South Korea relations." On 16 March 2015, Assistant Foreign Minister Liu Jianchao said during his visit to South Korea that "we hope that China’s concerns and worries will be respected." However, the next day, South Korean Ministry of National Defense spokesman Kim Min-seok expressed South Korea's dissatisfaction with China’s attitude, saying that "neighboring countries can have their own views on the possibility of the United States deploying the THAAD system in South Korea. But it should not try to influence our security policy." On 16 March 2015, U.S. Assistant Secretary of State Daniel R. Russel, who was visiting South Korea, also said that it was up to South Korea to decide "what measures to take and when to take them in its own joint defense", and also said that the US military "has a responsibility to consider what system" can protect Americans and their allies from the threat of North Korean missiles.

In addition to using the THAAD system, South Korea was also developing its own L-SAM long-range surface -to-air missile, which can intercept ballistic missiles flying at an altitude of 50 to 60 kilometers. However, the Defense Acquisition Program Administration (DAPA) of South Korea stated that the L-SAM system would not be deployed until 2023 to 2024. In May 2014, South Korea stated that it would not consider deploying the THAAD system and emphasized its own development of L-SAM. However, on 1 February 2016, it changed its attitude and expressed interest in the THAAD system. The South Korean Ministry of National Defense stated that its own missile, combined with the US anti-missile system, would become a powerful defense measure.

THAAD system

The Theater High Altitude Defense Missile (THAAD) is a defense system developed by Lockheed Martin. Production began in 1992, and the first contract was signed with the U.S. government in 2007. The main purpose of the THAAD system is to defend against short to medium-range ballistic missiles and can destroy harmful targets inside and outside the Earth's atmosphere. As of March 2017, the United States had deployed five THAAD anti-missile systems, one in Guam and the other four at Fort Bliss Army post in Texas. One THAAD system consists of a mobile launcher mounted on a truck, eight interceptors that can be launched and quickly reloaded, and a transportable radar surveillance system. The THAAD system deployed in South Korea this time cost about ₩1.5 trillion won per gun emplacement. South Korea was responsible for providing the base and infrastructure, while the United States was responsible for the specific deployment, operation and maintenance costs. Arms control expert Jeffrey Lewis said that South Korea needs to deploy two THAAD systems to fully cover its territory. As far as the current deployment location is concerned, North Korean submarines can launch attacks from the East China Sea, which is beyond the radar range. The interceptor missiles of the anti-missile system have a maximum range of 200 kilometers. The current deployment range includes several major U.S. military bases, but does not cover the capital Seoul.

The THAAD deployment

Consideration and commencement of THAAD consultations On 6 January 2016, North Korea conducted its fourth nuclear test, which it claimed was a hydrogen bomb test. A week later, South Korean President Park Geun-hye urged China to abide by its commitment to stop North Korea’s actions. In a televised address to the nation, she said, "China has repeatedly said publicly that it would not tolerate North Korea’s nuclear weapons. I think China is fully aware that if such strong will is not matched by necessary measures, we cannot prevent fifth and sixth nuclear tests by the North or guarantee real peace and stability on the Korean Peninsula." According to Yonhap News Agency, Park Geun-hye said at a press conference that day that, for reasons of national security and interests, the South Korean government would study the matter of introducing the THAAD system. Commentators generally believe that South Korea is making a statement and putting pressure on China, as if China does not take decisive measures on the North Korean nuclear issue, South Korea would deploy the THAAD system. On 29 January 2016, South Korean Ministry of National Defense spokesman Kim Min-seok said, "Our government will consider all measures to deal with the North Korean missile threat. If the U.S. military prompts South Korea to deploy the THAAD system, it will help our national security and defense." He also denied that South Korea had negotiated with the U.S. government. Some media reports pointed out that news showed that Lockheed Martin had visited South Korea in the past month.

On 7 February 2016, North Korea launched the Kwangmyŏngsŏng-4 Earth observation satellite at 8:34 a.m. local time and put it into orbit. This launch was widely regarded as a sign that North Korea had made significant progress in developing intercontinental ballistic missile (ICBM) technology because it shared similarities with the launch vehicle technology used to launch the satellite. Just hours after North Korea launched the satellite, Yoo Jae-seung, director of the Defense Policy Office of the South Korean Ministry of National Defense, announced at a press conference that South Korea and the United States had officially begun discussions on the early deployment of the THAAD system in South Korea. General Curtis Scaparrotti, commander of the U.S. Forces Korea, said in a statement that "North Korea continues to develop their nuclear weapons and ballistic missile programs, and it is the responsibility of our alliance to maintain a strong defense against those threats. THAAD can add an important capability in a layered and effective missile defense."

Deployment On 8 July 2016, the two countries officially announced the implementation of the THAAD anti-missile system. On 30 September 2016, South Korea announced that the THAAD system would be deployed at the Seongju Golf Course in Seongju County, North Gyeongsang Province, 296 kilometers from Seoul, replacing the previously planned Seongsan Fort in Seongju County. The Ministry of National Defense stated that the location in Seongju County would maximize the effectiveness of the THAAD system and minimize its impact on the public and the environment. The political scandal and the subsequent impeachment of Park Geun-hye at the end of October 2016 changed the course of this plan. On 21 December 2016, South Korean Prime Minister and Acting President Hwang Kyo-ahn called for the swift deployment of the latest THAAD system from the United States, stating, "As we cannot wait even for a moment to cope with North Korea's nuclear provocations, we have to do what we can do first."