South Korea – United States relations ( ) refers to international relations between South Korea and the United States . Relations commenced 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 . Relations between the United States and South Korea strengthened under the conservative , pro-American 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 2012 , South Korea was designated a major non-NATO ally . According to academics David Shambaugh and Michael Yahuda , there are currently several security factors shaping the alliance :
2009 G20 London summit
2009 G20 London summit
South Korea–United States relations