The United States deployed nuclear weapons on the Korean Peninsula between 1958 and 1991. Most of these weapons were intended to deter a ground invasion from North Korea by providing capabilities needed to slow or stop advancing troops and by convincing North Korea that any attack would invite unacceptable damage on the North in retaliation. Their presence was also meant to reassure South Korea of the U.S. commitment to South Korea's defense. The United States removed these weapons as a part of a broader change in the U.S. nuclear force posture at the end of the Cold War, but it remains committed to defending South Korea under the 1953 Mutual Defense Treaty and to employing nuclear weapons, if necessary, in that defense. Recent advances in North Korea's nuclear and missile programs have led to discussions, both within South Korea and, reportedly, between the U.S. and South Korean officials, about the possible redeployment of U.S. nuclear weapons on the Korean Peninsula. Some opposition leaders and members of South Korea's parliament have called for South Korea to develop its own nuclear weapons. More recently, in August 2017, representatives of the largest opposition party, Liberty Korea, called on the current government to ask the United States to redeploy U.S. nuclear weapons on the peninsula and suggested that they would push for approval of the nuclear deployments in the South Korean legislature and in discussions with U.S. officials. In addition, according to recent reports, South Korea's Defense Minister Song Young-moo may have indicated during discussions with U.S. Secretary of Defense Mattis that "redeployment of tactical nuclear weapons is an alternative worth a full review." South Korea's President Moon Jae-in has not supported proposals to redeploy U.S. nuclear weapons, and has continued to call for the denuclearization of the Korean Peninsula. South Korean government officials recently reiterated this stand, despite of the pressure from Liberty Korea and reports of some interest from U.S. officials. The U.S. government has not addressed, in public, the possible redeployment of nuclear weapons to the Korean Peninsula. However, press reports indicate that the Trump Administration is reviewing a wide range of military, diplomatic, and economic responses, and "is not ruling out moving tactical nuclear weapons to South Korea should Seoul request them." Senator John McCain, in an interview on CNN, also suggested the United States should consider redeploying nuclear weapons to the peninsula. Although some U.S. analysts have questioned whether the U.S. nuclear posture is sufficient to deter North Korea, most argue that, even if the United States sought to supplement its capabilities, it would not need to deploy nuclear weapons on the peninsula to deter or respond to North Korean aggression. Many also argue that the United States could expand its conventional capabilities and bolster its political commitments to South Korea to address concerns about its commitment to the U.S.-ROK alliance. Nevertheless, some have questioned whether the redeployment of nuclear weapons on the Korean Peninsula might be necessary if North Korea continues to expand its nuclear arsenal. As this debate unfolds, Congress could be asked to consider a number of questions about the potential costs and benefits of the redeployment of U.S. nuclear weapons to the Korean Peninsula, along with the implications for regional security and U.S.-allied relations. This In Brief report provides background information, explores the options that might be available, and discusses these potential implications. During the Cold War, nuclear weapons were central to the U.S. strategy of deterring aggression against the United States and U.S. allies in Europe and Asia. Toward this end, the United States deployed a wide variety of systems that could carry nuclear warheads. The long-range land-based intercontinental ballistic missiles (ICBMs), submarine-launched ballistic missiles (SLBMs), and heavy bombers that could carry cruise missiles or gravity bombs are known as strategic weapons. These were the weapons that the United States deployed so that it could threaten destruction of an adversary's central military, industrial, and leadership facilities. The United States also deployed thousands of shorter-range systems--including nuclear mines, artillery, short- and medium-range missiles, and some gravity bombs--on the territories of its allies, on naval vessels deployed around the world, and with its troops in the field. These weapons, which usually had less explosive power and were deployed with launchers that would deliver them across shorter ranges than strategic nuclear weapons, are known as nonstrategic, or tactical, nuclear weapons. They were intended for use by troops on the battlefield or within the theater of battle to achieve more limited objectives. On October 1, 1953, shortly after the end of the conflict on the Korean Peninsula, the United States and South Korea (officially known as the Republic of Korea, or ROK) signed a Mutual Defense Treaty, which provides that if either party is attacked by a third country, the other party will act to meet the common danger. To meet its obligations under this treaty, the United States has maintained a continuous military presence on the Korean Peninsula and is committed to helping South Korea defend itself, particularly against any aggression from the North. At the present time, the United States maintains about 28,500 troops in South Korea, while South Korean armed forces total over 625,000 troops, with about 490,000 in the Army, 70,000 in the Air Force, and 65,000 in the Navy. The United States deployed nuclear weapons in South Korea between 1958 and 1991. It deployed a number of different types of systems for use on the battlefield, including nuclear landmines, nuclear shells for howitzers, short-range surface-to-surface missiles, short-range cruise missiles, and some medium-range systems that could reach further into North Korea. These included nuclear bombs for delivery by fighter-bombers and the dual-mission Nike Hercules anti-air and surface-to-surface missiles. The number of nuclear weapons on the peninsula eventually reached nearly 950 warheads in the mid-1960s. Some of these weapons were deployed for just a few years, while others stayed for decades. Most of these weapons were intended to deter a ground invasion from North Korea, both by providing capabilities needed to slow or stop advancing troops and by convincing North Korea that any attack would not only be unsuccessful but would also invite unacceptable damage on the North in retaliation. In the mid-1970s, the Pentagon conducted a major review of the security of U.S. nuclear weapons storage sites in the Pacific and found that, in some cases, security was unsatisfactory and that the number of weapons deployed exceeded the requirements of the war plans. As a result, the United States began to withdraw some of its weapons systems; by 1977, it had deactivated the nuclear weapons storage facility at Osan Air base just south of Seoul, leaving nuclear weapons only at the base at Kunsan Air Base in the southwestern part of the country. As a result of these reductions, the number of nuclear weapons on the peninsula declined from around 540 in 1976 to approximately 150 artillery shells and bombs in 1985. By 1991, approximately 100 warheads remained, including around 60 shells for 155-mm Howitzer nuclear artillery and about 40 B61 bombs for delivery by fighter-bombers. On September 27, 1991, U.S. President George H. W. Bush announced that the United States would withdraw all of its land-based tactical nuclear weapons from overseas bases and all sea-based tactical nuclear weapons from U.S. surface ships, submarines, and naval aircraft. This initiative was a response to changes in the international security environment. In Europe, the threat the weapons were to deter--Soviet and Warsaw Pact attacks in Europe--had diminished with the collapse of the Warsaw Pact in 1989. Further, the military utility of the land-based weapons had declined. In Europe, the Soviet Union had pulled its forces eastward, beyond the range of these weapons. In Asia, growing U.S. and ROK conventional capabilities were seen as a more credible and capable response to the threat of an invasion from North Korea. While North Korea's nascent nuclear program was a concern, North Korea had not yet developed or tested functional nuclear weapons. The United States would rely on its longer-range, strategic nuclear weapons, deployed on land in the United States and on submarines at sea, and bomber aircraft deployed in the United States to deter North Korean aggression and to retaliate, if necessary, after an attack. The United States quickly removed its remaining 100 nuclear weapons from bases in South Korea--all were gone by the end of 1991. In addition, although the United States retained the B61 bombs in storage in the United States, it dismantled all the warheads for land-based shorter-range missiles and artillery. The U.S. Army no longer maintained the capability to deploy or employ nuclear weapons. Because the United States retired all warheads for land-based nonstrategic nuclear weapons under the 1991 presidential initiative, the only warheads remaining in the U.S. stockpile that could be deployed on the Korean Peninsula are B61 bombs, which can currently be delivered by B-2 bombers and F-15 or F-16 fighters. F-35 fighters, a next-generation strike fighter being procured for the U.S. military, may also eventually be equipped to deliver B61 bombs. To redeploy B61 bombs to South Korea, the United States would have to recreate the infrastructure needed to house the bombs. This would likely require the construction of secure vaults similar to those used to house U.S. nuclear weapons at bases in Europe, and the installation of similar security perimeters and safety systems. It would also require resources and training time to certify that the personnel at the bases were capable of maintaining the weapons and operating the aircraft for the nuclear mission. This would entail initial costs, when the bases are first modified and certified to store and handle nuclear weapons, and ongoing costs in time and training to maintain nuclear certification. While it is possible that the Pentagon could add resources to the Air Force to support a nuclear mission on the Korean Peninsula, the Air Force may face limits in funding and personnel, forcing it to shift needed resources from ongoing conventional missions in Korea to train the crews and maintain the weapons. As an alternative, the United States could reacquire and redeploy shorter-range nuclear-armed land-based missiles or artillery. This option could take a significant number of years to implement, as the U.S. Army no longer maintains or deploys missiles armed with nuclear warheads and the National Nuclear Security Administration retired the warheads that could be deployed on those missiles years ago. Recreating such a system would likely be expensive, as would be the effort to reintroduce nuclear weapons into the Army's forces. The time and resources needed to store, safeguard, and maintain the weapons, along with the time and resources needed to train and certify the troops, would likely far exceed those needed to return B61 bombs to the Korean Peninsula. Moreover, such an option would indicate that the United States and ROK viewed nuclear weapons as a complement to conventional forces on the battlefield, rather than as a deterrent to attack and a response to North Korea's nuclear program. Most of the discussions about the possible redeployment of U.S. nuclear weapons to South Korea focus on the role that these weapons might play in both deterring a North Korean attack against the South and reassuring South Korea of the U.S. commitment to the ROK's defense. As was noted above, the United States currently maintains nuclear weapons on land-based missiles and bombers in the United States and on submarine-launched ballistic missiles for this purpose. The United States also employs other strategic assets to demonstrate its commitment to South Korea's defense; it rotates B-1, B-2 and B-52 bombers through Andersen Air Force Base in Guam and periodically includes them in exercises with Japanese and South Korean aircraft. One such exercise occurred in late August, when two B-1Bs from Andersen Air Force Base (on the U.S. territory of Guam) and four U.S. Marine F-35Bs conducted a joint mission with four South Korean F-15K fighters. According to the U.S. Pacific Command, "this mission was conducted in direct response to North Korea's intermediate-range ballistic missile launch." While none of these aircraft carry nuclear weapons during these exercises, and the B-1 bombers are no longer capable of carrying nuclear weapons at all, many see the highly visible presence of U.S. strategic bombers as a clear indication of the power that the United States could bring to bear in support of South Korea. In recent years, the United States has also increased the frequency and depth of official interactions between U.S. and South Korean defense officials to bolster South Korea's confidence in the U.S. commitment to its defense. Most of these discussions have taken place in the Extended Deterrence Strategy and Consultative Group, where discussions help "build a better common understanding of extended-deterrence issues." The two nations have also held meetings at the ministerial level, with the U.S. Secretaries of Defense and State meeting with the ROK Foreign and Defense Ministers. The U.S. State Department recently announced that the United States and South Korea would hold more regular routine meetings of the Extended Deterrence Strategy and Consultative Group and would link these meetings more closely to the meetings at the ministerial level. Many analysts note that these meetings, along with the broader political relationship between the United States and South Korea, are critical to addressing South Korea's concerns. Few have questioned whether United States and ROK have the military capabilities that would be needed to defeat North Korea, although most note that the cost of a conventional conflict would be high, but, as is noted below, some see President Trump's comments about South Korea as a source of doubt about the U.S. commitment. Further, some have wondered whether the United States would be willing to fight on behalf of South Korea if it were vulnerable to a nuclear attack from North Korea. This is a classic dilemma in alliance relations known as "decoupling," which was often addressed during the Cold War with changes in the U.S. nuclear force posture. Within this context, and given that the only near-term option available is the possible redeployment of B61 bombs, the primary question is whether the U.S. ability to deter North Korean aggression and reassure South Korea of the U.S. commitment to its security is better served with the deployment of B61s on the peninsula, or whether their delivery by aircraft based in the United States is sufficient. Some who support the redeployment of U.S. nuclear weapons argue that their presence on the peninsula would be a visible, high-profile form of reassurance. They argue that, while off-shore weapons could conduct the necessary attacks if a conflict occurred, the act of returning weapons to the peninsula would send a powerful deterrent message to the North and demonstrate a strong commitment to the South. Some also argue that weapons located on the peninsula could serve as a "bargaining chip" in negotiations to freeze or eliminate North Korea's nuclear program, and that their presence would allow for a more rapid nuclear response to a North Korean attack. This is because aircraft based on the peninsula, if they were placed on alert during a crisis and loaded with nuclear weapons, could reach targets more quickly than aircraft coming from Guam or the United States. Those who oppose the redeployment of B61 bombs argue that the costs and risks of deployment on the peninsula would outweigh the benefits. Some note that nuclear weapons deployed on the peninsula could undermine deterrence and would present a tempting target for North Korea, possibly inviting an early attack if North Korea believed it needed to destroy the weapons before a conflict escalated to nuclear use. Moreover, although the time needed to deliver weapons to target would be shorter than for weapons based in the United States, some argue that this is unnecessary unless the United States planned to use the weapons to blunt an ongoing offensive. If the goal were to retaliate with unacceptable consequences after an attack, and not seek to disrupt the attack while it was ongoing, then the added time needed to launch the attack could be acceptable. In addition, the Pentagon could move existing weapons to a higher level of readiness during a crisis, which would shorten their time to use if they were needed to disrupt a North Korean attack. Finally, some assess that the cost of installing the necessary storage, security, and safety infrastructure could drain funding from other military priorities and time needed to train and certify the crews could undermine readiness for other military missions. If the United States believed it needed the capability to deliver nuclear weapons to North Korea in a shorter amount of time than allowed by the current force posture, it could pursue sea-based options that would not impose many of the costs or risks associated with the deployment of nuclear weapons on the peninsula. Although the United States deactivated and retired all its nuclear-armed sea-launched cruise missiles and carrier-based bombs after the 1991 presidential decision, it could, in time, reacquire and redeploy similar capabilities to add to its options in the Pacific. While the acquisition of these capabilities would impose financial and operational costs on the Navy, they are less likely to introduce attractive targets that could undermine stability on the peninsula itself. If the United States deployed its own nuclear weapons on the Korean Peninsula, it would not violate any agreements between the United States and South Korea. It would also not violate any existing agreements limiting U.S. nuclear weapons, unless the United States were to develop and deploy a land-based missile with a range between 500 and 5,500 kilometers. Land-based systems in this range are prohibited by the 1987 Intermediate-range Nuclear Forces (INF) Treaty. Neither air-delivered weapons, such as the B61 bomb, nor sea-based missiles with ranges between 500 and 5,000 kilometers, are limited by the treaty. Thus, neither would be limited or prohibited. The Nuclear Non-proliferation Treaty (NPT) does not prohibit the United States from basing its nuclear weapons in other countries. However, as with the pre-1991 period, the United States would need to retain operational control of the weapons at all times. Although President Moon has advocated for more muscular defense options, including reversing his decision to delay the deployment of a U.S. missile defense battery and requesting that the United States allow South Korea to develop more missile capabilities, his administration has indicated that it continues to support the denuclearization of the peninsula and that it is not currently reviewing its policy on the redeployment of U.S. nuclear weapons. The Liberty Korea Party, the main opposition party, has formally called for the move. As the party of the disgraced previous president, Liberty Korea is attempting to rebrand itself, including in the national security sphere. The position is relatively popular: In an early 2017 poll, over half of South Koreans favored South Korea developing its own nuclear weapons. A more recent survey released by Embrain reported that 68.4% percent of 1,015 respondents said the country needs to be armed with tactical nuclear weapons and nuclear-powered submarines, although the poll did not specify whether they preferred their own weapons or the redeployment of U.S. weapons. As one analyst points out, "Even if the North has effectively become a nuclear power, acknowledging it as one without South Korea itself going nuclear is politically untenable." The decision of whether to allow nuclear weapons back into South Korea could present Korean officials with both political and strategic problems. For those who maintain hope that North Korea could take steps toward denuclearization, including individuals in the current Moon administration, reintroducing nuclear weapons into South Korea could make it much more difficult to pressure the North to take these steps. As noted above, however, others argue that South Korea could use the presence of tactical nuclear weapons as a bargaining chip with the North; in other words, removal of such weapons could be offered as an incentive for North Korea to make its own concessions on the path to denuclearization. The redeployment of U.S. tactical nuclear weapons to the Korean Peninsula would likely be met with mixed reactions from the security community and the public in the ROK. On one hand, the move would reinforce the U.S. commitment to defend South Korea from the North, including the use of nuclear weapons, and would be welcomed by some South Koreans. Particularly among older South Koreans, many of whom remember the Korean War, there has long been a fear of U.S. abandonment in the face of North Korea's escalating threats. Stationing nuclear weapons, likely at the Osan base south of Seoul, may reassure those South Koreans concerned about the durability of the U.S. commitment. On the other hand, any change in the status quo could alarm Pyongyang, which could upset an already delicate security situation and unnerve the South Korean public. President Moon came into office pledging to reduce tensions with the North. Former U.S. ambassador to South Korea Mark Lippert points out that positioning nuclear weapons in the South would undercut the South's moral authority in calling for Pyongyang to disarm as well as dilute the U.S. goal of a denuclearized peninsula. The U.S.-ROK alliance is already feeling strains under the Moon and Trump Administrations. As candidates, both men made statements that observers believe undercut the strength of the alliance. As a candidate, Trump stated, "The countries we are defending must pay for the cost of this defense, and if not, the U.S. must be prepared to let these countries defend themselves." Candidate Trump also suggested he would be open to South Korea and Japan acquiring their own nuclear weapons, which would reverse decades of U.S. policy. President Moon, elected in May in a special election after the former president was impeached, campaigned on leftist themes, including criticizing the previous administration's decision to accept a U.S. missile defense battery known as the Theater High Altitude Area Defense system, or THAAD. As President, Trump asserted that South Korea should pay for the THAAD battery, contrary to the original agreement, thereby strengthening suspicious among some in South Korea that the United States would pin the cost of the system on Seoul. Although the two leaders have adjusted their positions and held a cordial summit in June 2017 that reiterated previous commitments to the alliance, cost-sharing and other flashpoints remain that could weaken the partnership in the months to come. Seoul has also expressed concern about President Trump's rhetoric mentioning a possible preventive military strike against North Korea, leading Moon to warn the United States in a nationally televised speech that no military action should be taken without South Korea's agreement. Moon's statement that "[o]nly the Republic of Korea can make the decision for military action on the Korean Peninsula" suggested to some observers that alliance coordination had become more contentious under these two administrations. A U.S. initiative to reintroduce tactical nuclear weapons could exacerbate differences between Washington and Seoul if the move is not carefully coordinated, including in terms of how it is presented to South Korea's public. Redeploying nuclear weapons to the Korean Peninsula could have security repercussions for the broader region as well. To China, this move would likely be seen as escalatory. Given Beijing's fierce criticism of the THAAD deployment, China likely would see further U.S. offensive capabilities in the region as threatening to its own security. The March 2017 announcement that THAAD was being deployed prompted a stern response from China, which warned that it would "take the necessary steps to safeguard our own security interests, and the consequences will be shouldered by the United States and South Korea." Chinese state media encouraged Chinese consumers to boycott South Korean companies and tourism officials said that they would cease booking trips to South Korea by Chinese travelers, retaliations with serious consequences for many South Korean companies who depend heavily on the Chinese market. Some analysts, however, argue that demonstrating U.S. resolve could pressure China to exert more influence on the Pyongyang regime to change its behavior. In the past, China appears to have been inclined to put more pressure on North Korea when the United States has pursued measures that bolster U.S. capabilities in the region, such as enhancements to American missile defense systems, in response to DPRK provocations. On the other hand, Beijing may be sufficiently threatened by the presence of U.S. nuclear weapons that it instead increases its support to North Korea. China could also respond by increasing its own nuclear weapons arsenal. In addition, because of the complexity and range of issues that frame the U.S.-China bilateral relationship, other U.S. interests in the region could be affected, including trade, maritime disputes in the South China and East China Seas, Taiwan, and human rights concerns. For Japan, reaction could be mixed as well. Tokyo is currently engaged in a debate about whether Japan should upgrade its own defense capabilities in light of the increased threat from North Korea. As with Seoul, the United States has a mutual defense treaty with Tokyo that includes the nuclear "umbrella." However, no U.S. nuclear weapons have been based in Japan, with the exception of U.S. naval ships carrying nuclear weapons into Japanese ports during the Cold War under a secret agreement from the 1960s. Given Japan's sensitivity to the use of any nuclear power--based on its history as the only country bombed by an atomic weapon and further demonstrated by resistance among many Japanese to using nuclear reactors for electricity generation--it seems highly unlikely that Japan would seek U.S. nuclear weapons on its own soil. Nevertheless, any adjustment of the U.S. military posture on the peninsula, including a redeployment of nuclear weapons to the peninsula, could create additional security concerns for Tokyo if it sees the moves as bolstering South Korea's military capabilities. These concerns would likely be heightened by the historical rivalry between Tokyo and Seoul. The Department of Defense is currently conducting a Nuclear Posture Review that is examining both the role of nuclear weapons in U.S. national security policy and ongoing plans to modernize the U.S. nuclear enterprise. While it is not known, at this time, whether the review will address questions about the overseas deployment of U.S. nuclear weapons or recommend any changes to those deployments, the public debate in South Korea about the possible redeployment of U.S. nuclear weapons could affect the review's recommendations. If the review recommends changes in the U.S. force posture in Asia, either through the deployment of weapons on the peninsula, or through the development of new types of weapons to reinforce deterrence in the region, Congress may review not only the implications of that decision, but also the costs associated with developing and deploying new weapons. Congress may also consider how other U.S. policies and deployments might bolster South Korea's confidence in the United States' commitment to its defense. These steps could include increased deployments of U.S. nonnuclear strategic assets to South Korea, changes in military exercises, and the expansion of U.S.-ROK consultation strategic consultations.

Summary:
Recent advances in North Korea's nuclear and missile programs have led to discussions, both within South Korea and, reportedly, between the United States and South Korean officials, about the possible redeployment of U.S. nuclear weapons on the Korean Peninsula. The United States deployed nuclear weapons on the Korean Peninsula between 1958 and 1991. Although it removed the weapons as a part of a post-Cold War change in its nuclear posture, the United States remains committed to defending South Korea under the 1953 Mutual Defense Treaty and to employing nuclear weapons, if necessary, in that defense. The only warheads remaining in the U.S. stockpile that could be deployed on the Korean Peninsula are B61 bombs. Before redeploying these to South Korea, where they would remain under U.S. control, the United States would have to recreate the infrastructure needed to house the bombs and would also have to train and certify the personnel responsible for maintaining the weapons and operating the aircraft for the nuclear mission. Some who support the redeployment of U.S. nuclear weapons argue that their presence on the peninsula would send a powerful deterrent message to the North and demonstrate a strong commitment to the South. Their presence would allow for a more rapid nuclear response to a North Korean attack. Some also argue that weapons could serve as a "bargaining chip" with North Korea and that their presence would allow for a more rapid nuclear response to a North Korean attack. Those who oppose the redeployment argue the weapons would present a tempting target for North Korea and might prompt an attack early in a crisis. They also argue that nuclear weapons based in the United States are sufficient for deterrence, and that the costs of installing the necessary facilities on the peninsula could detract from conventional military capabilities. Finally, some assess that the cost of installing the necessary storage, security, and safety infrastructure could drain funding from other military priorities and time needed to train and certify the crews could undermine readiness for other military missions. Some analysts also assert that, if the United States believed it needed the capability to deliver nuclear weapons to North Korea in a shorter amount of time than allowed by the current force posture, it could pursue sea-based options that would not impose many of the costs or risks associated with the deployment of nuclear weapons on the peninsula. South Korea's President Moon Jae-in has advocated for more muscular defense options, but does not support the redeployment of U.S. tactical nuclear weapons. The Liberty Korea Party, the main opposition party, has formally called for the move. While some in South Korea believe nuclear weapons are necessary to deter the North, others, including those who maintain hope that North Korea will eliminate its program, argue that their redeployment could make it that much more difficult to pressure the North to take these steps. Further, if North Korea saw the deployment as provocative, it could further undermine stability and increase the risk of conflict on the peninsula. China would also likely view the redeployment of U.S. nuclear weapons as provocative; it has objected to U.S. military deployments in the past. Some analysts believe that China might respond by putting more pressure on North Korea to slow its programs, while others believe that China might increase its support for North Korea in the face of a new threat and, possibly, expand its own nuclear arsenal. Japan's reaction could also be mixed. Japan shares U.S. and South Korean concerns about the threat from North Korea, but given its historical aversion to nuclear weapons, Japan could oppose the presence of U.S. nuclear weapons near its territory. In addition, any adjustment of the U.S. military posture on the peninsula could create additional security concerns for Tokyo.