Last Updated: October, 2018
South Korea, officially the Republic of Korea (ROK), has maintained a bilateral security alliance with the United States since the Korean War (1950-1953). The ROK abandoned its nuclear weapons efforts in the 1970s, and is engaging in diplomatic efforts with North Korea and the United States to attempt to denuclearize the Korean peninsula. South Korea is a party to all major nonproliferation treaties and regimes, possesses ballistic and cruise missiles, and cooperates with the United States on missile defense.
South Korea first became interested in nuclear technology in the 1950s, but did not begin construction of its first nuclear power plant until 1971.  Changes in the international security environment influenced South Korea's decision to begin a nuclear weapons program in the early 1970s, but the ROK abandoned the effort under significant pressure from the United States.  In April 1975, South Korea signed the Treaty on the Non-Proliferation of Nuclear Weapons (NPT).  South Korea is also a party to the Comprehensive Nuclear Test Ban Treaty (CTBT) and is a member of the Nuclear Suppliers Group and the Zangger Committee. 
The United States deployed tactical nuclear weapons in South Korea from 1958 until 1991.  Afterward, South Korea focused its diplomatic efforts on keeping the Korean peninsula free of nuclear weapons. In January 1992, North and South Korea signed the Joint Declaration on the Denuclearization of the Korean Peninsula. In this agreement, North Korea and South Korea agreed not "to test, manufacture, produce, receive, possess, store, deploy or use nuclear weapons," and not to "possess nuclear reprocessing and uranium enrichment facilities."  However, both sides failed to implement the agreement's provisions relating to a bilateral inspections regime.  Despite North Korea’s violations of the Joint Declaration, including its multiple nuclear weapons tests, South Korea has never officially renounced its obligations under the declaration, and has called on the North to abide by the agreement. 
Beginning in 2003, South Korea participated in the Six-Party Talks, aimed at ending the nuclear crisis on the Korean Peninsula, until North Korea withdrew in 2009 over disagreements with the UN Security Council.  At the April 2018 Inter-Korean Summit, North and South Korean heads of state declared their aspiration for a “complete denuclearization” of the Korean Peninsula, echoing sentiments expressed 26 years earlier. 
South Korea’s civil nuclear sector is highly developed; 23 nuclear reactors supply a third of the country’s electricity. Two additional reactors are under construction.  Following President Moon Jae-in’s June 2017 announcement that South Korea would limit its use of nuclear energy moving forward, South Korea froze all ongoing construction and scrapped plans for future reactors.  Facing considerable public opposition, the Moon government restarted work on the two partially completed nuclear reactors in October 2017, but still plans to phase out nuclear power over the coming decades. 
South Korea ratified the Biological and Toxin Weapons Convention (BTWC) in June 1987 and joined the Australia Group in October 1995.  Although South Korea possesses a well-developed pharmaceutical and biotechnology infrastructure, it has committed to pursue only defensive BW research and development. 
South Korea ratified the Chemical Weapons Convention (CWC) in April 1997.  At that time, South Korea reportedly confidentially declared possession of several thousand metric tons of chemical warfare agents and one chemical weapons (CW) production facility to the Organization for the Prohibition of Chemical Weapons (OPCW). Neither the OPCW nor South Korea has publicly acknowledged this declaration. The South Korean government has maintained a high level of secrecy regarding its previous chemical weapons activities, reportedly requiring the OPCW to refer to it in all documents as "another state party" or "an unnamed state party." However, media reports indicate that pursuant to its CWC obligations, the South Korean military built and operated a CW destruction facility to eliminate all CW munitions at a site in Yeongdong Chungcheong. 
Under the CWC, South Korea was obligated to eliminate its CW stockpile by April 2007. South Korea requested an extension on that deadline from the OPCW, reportedly citing a number of technical difficulties in the operation of its destruction facility. South Korea reportedly completed the destruction of its entire chemical weapons stockpile in July 2008, becoming the second CWC member to do. 
In the wake of the shelling of Yeonpyeong Island in November 2010, the South Korean government and the National Emergency Management Agency (NEMA) began to equip civilian facilities — such as subway stations — with gas and oxygen masks, as well as oxygen tanks, to be used in case of chemical attacks by North Korea.  Additionally, the U.S. chemical warfare battalion, which previously left South Korea in 2004, was redeployed to the Korean peninsula in 2013. 
South Korea began developing missiles in the early 1970s, and successfully tested its first missile system NHK-1 (aka Baekgom-1), in September 1978. Currently, South Korea deploys a series of short-range ballistic missiles and several types of cruise missiles. South Korea joined the Missile Technology Control Regime (MTCR) in March 2001.  Until 2017, South Korea’s missile program was constrained by a series of bilateral agreements with the United States, which imposed range and payload restrictions on its indigenous missiles. In response to North Korea’s Unha-3 rocket launch in 2012, the United States and South Korea issued a new memorandum allowing South Korea to develop missiles ranging up to 800 kilometers, far enough to strike targets anywhere in North Korea.  In response to North Korean missile and nuclear tests in 2017, the United States and South Korea also agreed to lift restrictions on missile payloads.  In the 1990s, South Korea began development of a space program, including a space-launch vehicle (SLV). After numerous delays, South Korea successfully launched the two-stage KSLV-1 rocket on August 25, 2009. South Korea successfully placed a satellite into orbit in January 2013. 
In early 2017, the United States deployed two Terminal High Altitude Aerial Defense (THAAD) systems to South Korea, supplementing existing deployments of Patriot and Aegis missile defense systems in the region.  Despite a campaign pledge to remove the missiles, the Moon administration decided to continue THAAD deployments following North Korea’s ICBM tests in 2017. 
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 Michael Elleman, "North Korea Launches Another Large Rocket: Consequences and Options," 38 North, 10 February 2016; Alex Johnson, Stella Kim, Courtney Kube, “U.S. Begins Shipping Controversial Anti-Missile System to South Korea,” NBC News, 7 March 2017, www.nbcnews.com.
Get the Facts on South Korea
- Operates 23 nuclear power reactors which provide 35% of its electricity
- Completed destruction of its chemical weapons stockpile in July 2008
- Owns a well-developed biotechnology infrastructure, but no evidence suggests the pursuit of a biological weapons program