U.S. interests in the Republic of Korea (R.O.K.—South Korea) involve security, economic, and political concerns. The United States suffered over 33,000 killed and over 101,000 wounded in the Korean War (1950-53). The United States agreed to defend South Korea from external aggression in the 1954 Mutual Defense Treaty. The Treaty obligates the United States and South Korea to (1) seek to settle international disputes "by peaceful means"; (2) refrain from "the threat or use of force" that is inconsistent with the purposes of the United Nations; (3) consult together when either party "is threatened by external armed attack" and resort to "mutual aid" and "appropriate means" to deter an armed attack; (4) "act to meet the common danger in accordance with its constitutional processes" if the territories of either party "in the Pacific area" are subject to "an armed attack." Under the Mutual Defense Treaty, South Korea grants the United States the rights to station U.S. military forces in South Korea "as determined by mutual agreement." The United States maintains about 28,000 troops there to supplement the 650,000-strong South Korean armed forces. This force is intended to deter North Korea's (the Democratic People's Republic of Korea—D.P.R.K.) 1.2 million-man army. Since 1991, U.S. attention has focused primarily on North Korea's drive to develop nuclear weapons. However, other North Korean policies and actions have affected U.S. interests including proliferation of missiles and other weapons of mass destruction to Middle Eastern countries, support for terrorist groups in the Middle East and South Asia, counterfeiting of U.S. currency and U.S. products, human rights abuses, and policies that have forced thousands of North Koreans to flee to China as refugees. North Korean policies are important issues in U.S. relations with China and Japan. The United States is South Korea's third-largest trading partner (replaced as number one by China in 2002) and second-largest export market. South Korea is the seventh-largest U.S. trading partner. Total trade is close to $80 billion annually. In 2007, the United States and South Korea signed a Free Trade Agreement (FTA). Neither President Bush nor President Obama has submitted the FTA to Congress for approval. If approved, it would be the second-largest U.S. FTA; only the North American Free Trade Agreement would be larger. The Clinton and Bush Administrations concentrated on North Korea's nuclear weapons program in their policies toward North Korea. The Obama Administration is continuing to prioritize the nuclear issue. Other issues, from North Korean missiles to human rights, have been subordinated. (For additional information on this subject, see CRS Report RL33590, North Korea ' s Nuclear Weapons Development and Diplomacy , and CRS Report RL34256, North Korea's Nuclear Weapons: Technical Issues .) On October 9, 2006, North Korea conducted its first nuclear test, a small plutonium explosion of less than one kiloton (3% to 4% of the explosion power of the Nagasaki plutonium atomic bomb). On May 25, 2009, North Korea announced that it had conducted a second nuclear test. Most estimates placed the strength of the explosion at between two and four kilotons. U.S. intelligence agencies and non-government experts estimate that North Korea has between 30 and 50 kilograms of nuclear weapons grade plutonium that it extracted from its operating five megawatt nuclear reactor at Yongbyon. Using six kilograms per weapon, this would be enough for five to eight atomic bombs. North Korea admitted in June 2009 that it has a program to enrich uranium, and it later claimed that the program was advancing. The United States had cited evidence of such a program since 2002. North Korea reportedly has proliferated nuclear technology in the Middle East. According to U.S. officials, North Korea collaborated with Syria in the construction of a nuclear reactor, which Israel bombed in September 2007. Numerous reports going back to 1993 describe extensive North Korean collaboration with Iran. This collaboration reportedly involves development of highly enriched uranium, development of a nuclear warhead that could be mounted on a jointly developed intermediate-range ballistic missile (the North Korean Nodong missile, named the Shahab by Iran); collaboration in developing missiles, and North Korean assistance in constructing deep underground installations to house part of Iran's nuclear installations. Since August 2003, negotiations over North Korea's nuclear weapons programs have involved six governments: the United States, North Korea, China, South Korea, Japan, and Russia. On April 14, 2009, North Korea terminated its participation in the six party talks and said it would not be bound by agreements between it and the Bush Administration, ratified by the six parties, which partially disabled the Yongbyon facilities. North Korea also announced that it would reverse the disablement process under these agreements and restart the Yongbyon nuclear facilities. Three developments since August 2008 appear to have influenced the situation leading to North Korea's announcement: the failure to complete implementation of the Bush Administration-North Korean agreement, including completing the Yongbyon disablement, because of a dispute over whether inspectors could take samples of nuclear materials at Yongbyon; the stroke suffered by North Korean leader Kim Jong-il in August 2008; and the issuance by North Korea after January 1, 2009, of a tough set of negotiating positions, including an assertion that the United States must extend diplomatic relations prior to any final denuclearization agreement rather than in such an agreement, that a denuclearization deal must include the construction of light water nuclear reactors in North Korea, and that U.S. reciprocity for North Korean denuclearization must be an end of the "U.S. nuclear threat," meaning major reductions of and restrictions on U.S. military forces in South Korea and around the Korean peninsula. The Obama Administration reacted to the missile and nuclear tests by seeking United Nations sanctions against North Korea. It secured U.N. Security Council approval of Resolution 1874 in June 2009. The resolution calls on U.N. members to restrict financial transactions in their territories related to North Korean sales of weapons of mass destruction (WMD) to other countries. The Security Council imposed sanctions on over a dozen North Korean trading companies and banks that operated in other countries to facilitate Pyongyang's WMD transactions. The resolution also calls on U.N. members to prevent the use of their territories by North Korea for the shipment of WMD to other countries. U.S. officials subsequently visited China and other countries in an effort to secure their cooperation in enforcing Resolution 1874, particularly the sanctions against financial transactions. China's role in enforcing sanctions is especially important because of the reported access of North Korean trading companies and North Korean banks to Chinese banks and the use of Chinese air space and airports by aircraft traveling between North Korea and Iran. In the initial months after passage of Resolution 1974, there is no evidence that China had adopted measures to end these North Korean proliferation activities. North Korea seemed to moderate its provocative policies beginning in August 2009. It invited former President Bill Clinton to North Korea, where he secured the release of two female American reporters who were taken prisoner by the North Koreans along the China-North Korea border. North Korea also took several apparent conciliatory actions toward South Korea, including lifting restrictions on the operations of South Korean companies at the Kaesong industrial complex inside North Korea, releasing a South Korean worker who had been arrested at Kaesong, and allowing a meeting of members of divided Korean families at the end of September 2009. North Korea's motives for this change of strategy and tactics were not totally clear, but one apparent motive was North Korea's worsening food situation and Pyongyang's decision to try to restore U.S. and South Korean food aid donations. North Korea had rejected South Korean food and fertilizer aid in 2008, and it expelled the U.S. food aid program in March 2009. South Korea welcomed several of the North Korean actions, but President Lee Myung-bak proposed a "grand bargain" on the nuclear issue which would lay out the steps toward total denuclearization of North Korea and provide North Korea with diplomatic, aid, and energy benefits in return. Lee said that South Korea would not extend significant amounts of aid to North Korea until Pyongyang made major concessions on the nuclear issue. North Korea's leader, Kim Jong-il (in apparently improved health after his August 2008 stroke), called in October 2009 for bilateral U.S.-North Korean negotiations aimed at ending U.S. "hostile policies." North Korean officials demanded that the Obama Administration end its promotion of U.N. sanctions. Kim Jong-il referred to a possibility of "multilateral talks" that could include six-party talks if the U.S.-North Korean bilateral talks produced the results sought by North Korea. The Obama Administration sent Ambassador Stephen Bosworth, the Administration's chief envoy on North Korea, to Pyongyang. Bosworth sought two commitments from North Korea: a commitment to resume participation in the six party talks and a renewed commitment to the September 2005 six party statement calling for denuclearization of the Korean peninsula. According to Bosworth, North Korea made general commitments to both of these. However, it made no commitment to a time when it would resume participation in the six party meetings. North Korean officials reportedly told Bosworth that the Obama Administration should lift U.N. and U.S. sanctions against North Korea and agree to negotiate a U.S.-North Korean peace treaty.