It's possible that you are referring to the country of South Korea, which the United States defended during the Korean War. In 1950, North Korea invaded South Korea, and the United States, along with other United Nations member countries, intervened to stop the aggression. The United States provided military support to South Korea and helped to push back against North Korean forces. Ultimately, an armistice was signed in 1953, which ended the fighting and established a demilitarized zone between North and South Korea.