Republic of Korea–United States relations ( Hangul : 한미 관계 ; Hanja : 韓美 關係 ; RR : Hanmi gwangye ; MR : Hanmi kwan'gye ) have been extensive since 1950, when the United States helped establish the modern state of South Korea , also known as the Republic of Korea, and fought on its UN -sponsored side in the Korean War (1950–1953). During the subsequent four decades, South Korea experienced tremendous economic , political and military growth, and significantly reduced U.S. dependency. From Roh Tae-woo 's administration to Roh Moo-hyun 's administration, South Korea sought to establish an American partnership, which has made the Seoul – Washington relationship subject to some strains, especially with the Anti-US/Korean sentiments. However, relations between the United States and South Korea have greatly strengthened under the conservative, pro-U.S. Lee Myung-bak administration. At the 2009 G20 London summit , U.S. President Barack Obama called South Korea "one of America's closest allies and greatest friends."  In addition, South Korea has been designated as a Major non-NATO ally . 
According to a 2014 BBC World Service Poll, 58% of South Koreans view the United States' influence positively, while 28% view it negatively; 55% of Americans view South Korea's influence positively, while 34% view it negatively. South Korea is one of the most pro-American nations in the world.
|Republic of Korea||United States of America|
|Coat of Arms|
|Area||100,210 km 2 (38,690 sq mi)||9,820,630 km 2 (3,791,770 sq mi)|
|Population Density||506/km 2 (1,310/sq mi)||35/km 2 (91/sq mi)|
|Largest City||Seoul – 10,464,051 (25,650,063 Metro)||New York City – 8,600,710 (19,006,798 Metro)|
|Government||Unitary presidential constitutional republic||Federal presidential constitutional republic|
|First Leader||Rhee Syng-man||George Washington|
|Current Leader||Moon Jae-in||Donald Trump|
|Official languages||Korean||English ( de facto , none at federal level)|
|GDP (nominal)||US$ 1.450 trillion ($28,739 per capita )||US$16.245 trillion ($51,704 per capita)|
Leaders of South Korea and the United States from 1950
Korean War (6.25 War)
Cross-border skirmishes and raids at the 38th Parallel escalated into open warfare when the North Korean forces invaded South Korea on June 25, 1950. The Korean War broke out when North Korea invaded South Korea . In response, 16 member countries of the United Nations , including the United States , came to the defense of South Korea . It was the first significant armed conflict of the Cold War with extensive deployment of American and other troops. 
Origins of the South Korea–United States alliance
Following the end of World War II, the United States established a bilateral alliance with South Korea instead of establishing a multilateral alliance with South Korea and other East Asian countries.
Moreover, the "U.S. alliance with South Korea would consequently have three functions. First, it would serve as part of a network of alliances and military installations designed to ring the Soviet threat in the Pacific. Second, it would deter a second North Korean attack, with U.S. ground troops serving as the "tripwire" guaranteeing U.S. involvement. Third, it would restrain the South from engaging in adventurism."
The Republic of Korea and the United States agreed to a military alliance in 1953.  They called it "the relationship forged in blood".  In addition, roughly 29,000 United States Forces Korea troops are stationed in South Korea. In 2009, South Korea and the United States pledged to develop the alliance’s vision for future defense cooperation.  Currently South Korean forces would fall under United States control should the war resume. This war time control is planned to revert to South Korea in 2020. 
At the request of the United States , President Park Chung-hee sent troops to Vietnam to assist American troops during the Vietnam War , maintaining the second largest contingent of foreign troops after the United States . In exchange, the United States increased military and economic assistance to South Korea. President Roh Moo-hyun , despite having been elected on a liberal platform, also authorized dispatching a small contingent of troops to Iraq in 2004 at the request of President George Bush.
Since the end of the Korean War, South Korea and the United States have maintained strong ties.
According Pew Research Center, 84% of South Koreans have a favorable view towards the United States and Americans (ranked within top 4 among the countries in the world).   Also, according to a Korean Gallup poll, South Korea views the U.S. as the most favorable country in the world.  In the political side, the United States supported South Korea after 1945 as a "staunch bastion against communism", even when it was ruled by a dictatorship.  In a March 2011 Gallup Poll, 74% of South Koreans said that they believe that the U.S. influence in the world is favorable,  and in a November 2011 Gallup Poll, 57% of South Koreans approved of U.S. leadership, with 22% disapproving; by contrast, only 30% of South Koreans approved of China 's leadership. 
Americans are steadily viewing South Korea more positively as well, with the 2011 Gallup poll – a 65% favorability rating – being the highest rating to date.  Thus, the relationship between the two countries, as indicated by polling results, is steadily improving.
On February 9, 2000, the Eighth U.S. Army ordered twenty boxes of formaldehyde , a toxic fluid, dumped into the Han River . South Korean environmentalist groups protested that it could be harmful to aquatic life forms, but the U.S. military insisted that it was diluted with water. This incident was satirized in the 2006 South Korean monster film The Host , where a horrible mutated monster from the river menaces the inhabitants of Seoul. 
The Government of South Korea banned imports of U.S. beef in 2003 in response to a case of mad cow disease in Washington state . In 2008, the protests against U.S. beef recalled the student "pro-democracy" movements of the 1980s. Nevertheless, South Korea became the world's third largest U.S. beef importer in 2010. With its strong import growth, South Korea surpassed Japan for the first time to become the largest market for U.S. beef in Asia and in 2016 US beef imports in Korea reached a value of $1 billion.  
South Korea and the United States are important economic partners to each other. Nearly 60 billion dollars of trade volume between the two countries display the significant economic interdependence between the two states. However, according to the CRS report, South Korea is much more economically reliant on the United States than the United States is on South Korea. This is supported with the fact that the United States ranks first as a trading partner for South Korea. However, a recent policy brief introduces the fact that the ratio of exports to the United States has declined significantly from around 40 percent to less than 20 percent in 2002 while the share of exports to China has increased drastically which led China to become the number one export destination for South Korea.  Although the economy of South Korea and the United States is becoming more integrated with the recent ratification of the KORUS Free Trade Agreement, there remains some major trade disputes between the two nations in the areas including telecommunications, automotive industry, intellectual property rights issues, pharmaceutical industry, and agricultural industry especially in terms of rice and beef.