North Korea was backed by the USSR and South Korea was backed by the USA. And a war erupted. Sounds familiar? Well, that happened a number of times during the Cold War.
During the first half of the twentieth century, Korea was under the control of Japan. However, at the end of the second world-war, since Japan had lost most of its power, USSR saw an opportunity there and liberated North Korea. And as the familiar story of the Cold War goes, USA did not stay behind and entered through South. By 1948, Korea had two separate governments for the north and the south – and neither acknowledged the other government.
There was a brief period of peace, but it soon ended in 1950 when USSR and (now also) China backed North Korea invaded South Korea. To this invasion, the United Nations reacted strongly and sent its military force. The war ended in 1953 (once again, not technically, as no peace has been signed), but that’s how the life goes on.