South Korea
United States

A close relationship with the United States began directly after World War II, when the United States temporarily administered Korea for three years (mainly in the South, with the Soviet Union engaged in North Korea). Upon the onset of the Korean War in 1950, U.S. forces were sent to defend against an invasion from North Korea of the South and subsequently fought as the largest contributor of UN troops. The United States participation was critical for preventing the near defeat of the Republic of Korea by northern forces, as well as fighting back for the territory gains that define the South Korean nation today.

Following the Armistice, South Korea and the U.S. agreed to a "Mutual Defense Treaty", under which an attack on either party in the Pacific area would summon a response from both. In 1967, South Korea obliged the mutual defense treaty by sending a large combat troop contingent to support the United States in the Vietnam War. The two nations have strong economic, diplomatic, and military ties, although they have at times disagreed with regard to policies towards North Korea and with regard to some of South Korea's industrial activities that involve usage of rocket or nuclear technology. There had also been strong anti-American sentiment during certain periods, which has largely moderated in the modern day.

The two nations also share a close economic relationship, with the U.S. being South Korea's second largest trading partner, receiving $66 billion in exports in 2016. In 2007, a free trade agreement known as the Republic of Korea-United States Free Trade Agreement was signed between South Korea and the United States, but its formal implementation was repeatedly delayed, pending approval by the legislative bodies of the two countries. On October 12, 2011, the U.S. Congress passed the long-stalled trade agreement with South Korea. It went into effect on March 15, 2012.
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