South and North Korea fought in a battle for 3 years starting from 1950, known as the Korean War. It began on 25 June 1950 because north Korea started invading the South. The United States and UN, aka the United Nations are on South Korea's side, while the USSR (Soviet Union), and China was on the other.
Korea was divided between America and the USSR after WW2, but because of tensions between those 2 nations, the 2 Korea's became sovereign states in 1948. Kim Il Sung was the leader of North Korea, in the capital Pyongyang and Syngman Rhee in South Kore's capital, Seoul. They both said that they were the government of all of Korea, and they would not agree to the borders.
Seoul was facing a communist party in Jeju, and after an unsuccessful attempt on unifying Korea as well as Seoul suppressing communists in Seoul, North Korea invaded South Korea on 25 June 1950. Then the UN partied with several other countries to help South Korea, with America making up the majority of the forces.
South Korea was certainly lost only after 2 months of the war, holding onto only the Pusan Perimeter. But in September on the same year, South Korea's allies like America and the UN landed at Incheon. *STAR SPANGLED BANNER INTENSIFIES* the forces punched into North Korea next to the Chinese border. As a result, China entered the war on North Korea's side. The forces retreated from north Korea because of the first and second Chinese attacks, and Seoul was taken on 1951 before it being free once again. They kept going back and forth, and it turned into a war of attrition.
On July 7, 1953, was when it officially ended when the Korean Armistice Agreement, which allows returned prisoners, and the making of the infamous demilitarized zone, the DMZ. 3 million people died and had more civilian deaths then WORLD WAR 2! But no treaty was ever made, so this is still not resolved.
Go to maps to take a closer look at the war!