South Korea (Republic of Korea or ROK) is one of the United States' most important strategic and economic partners in Asia. Since the early 1950s, the U.S.-ROK Mutual Defense Treaty commits the United States to help South Korea defend itself. Approximately 28,500 U.S. troops are based in the ROK, which is included under the U.S. "nuclear umbrella." Washington and Seoul cooperate in addressing the challenges posed by North Korea. The two countries' economies are joined by the Korea-U.S. Free Trade Agreement (KORUS FTA). South Korea is the United States' seventh-largest trading partner and the United States is South Korea's second-largest trading partner (China is its first-largest). Political changes in both countries in 2017 along with North Korea's increasing military capabilities have produced strains in the relationship. Coordination of North Korea Policy Dealing with North Korea is the dominant bilateral strategic concern. The Trump Administration appears to have raised North Korea's nuclear and missile programs to a top U.S. foreign policy priority, and has adopted an approach of increasing pressure on Pyongyang in the hopes of convincing the North Korean regime "to de-escalate and return to the path of dialogue." The Administration has emphasized pushing China, which accounts for around 90% of North Korea's trade, to do more to pressure North Korea. North Korea's two tests in July 2017 of long-range ballistic missiles, which some observers characterized as having intercontinental range, has heightened the U.S. sense of urgency over the North Korean issue. ROK President Moon Jae-in, elected in May 2017, has said he supports the continuation of sanctions against North Korea if it is aimed at bringing North Korea to the negotiating table. He also argued, however, against a "sanctions-only" approach toward North Korea. Instead, President Moon envisions denuclearization policy and low-level inter-Korean initiatives as complementary and in July 2017 proposed holding several inter-Korean dialogues. These initiatives appear to have caused some tension in U.S.-ROK relations, but North Korea's lack of a response to them and its continued missile testing appear to have ameliorated disagreements between Washington and Seoul, at least temporarily. The U.S.-ROK Alliance Since 2009, the United States and South Korea have accelerated steps to reform their alliance, including the relocation of U.S. troops on the Korean Peninsula and boosting ROK defense capabilities. Provocations from North Korea have propelled more integrated bilateral contingency planning, for instance by adopting policies to respond more swiftly and forcefully to attacks and by deploying the Terminal High Altitude Area Defense (THAAD) missile defense system in South Korea. According to U.S. military officials, South Korea pays roughly half of the non-personnel costs of stationing U.S. troops in South Korea. Many analysts think that the Trump Administration will demand that South Korea increase its cost-sharing payments. Bilateral Economic Relations The KORUS FTA has been the centerpiece of U.S.-Korea trade and investment relations since its entry into force in 2012, but views on KORUS FTA's economic outcomes are mixed. Most U.S. business groups highlight market access improvements and a more robust mechanism for dispute resolution, but others have raised concerns over implementation. The Trump Administration has repeatedly criticized the agreement, focusing on the growth in the bilateral trade deficit since its entry into force, and has called for modifications. Following a U.S. request, the two nations are expected to convene a special committee under the KORUS FTA to discuss the agreement and possible changes. The scope of these discussions remains unclear, as the Administration has yet to officially notify Congress of its intent to negotiate with South Korea, a requirement for any trade agreement negotiation to receive expedited legislative conside