After Japan surrendered in August 1945, the Soviet Union, under Stalin, and the USA, under Truman, divided Korea at the 38th parallel with the Soviets occupying the north and the USA the southern half.
Kim Il-Sung tried to reunite Korea under the communist government with the help of the Soviets.
On June 25, 1950, North Korea invaded South Korea, who was being led by Syngman Rhee.
The United Nations declared North Korea's attack illegal. They banded together to support South Korea by providing troops.
Many people died during the Korean War. From South Korea, around 1,217,000 died, 1,000,000 civilian and 217,000 military. As for North Korea, 1,006,000 died, 406,000 civilian and 600,000 military.
In the end, no country won the war. Till this day, North Korea remains a communist country, while South Korea remains a democratic country.