The Korean War was caused by the conditions holding since the end of World War Two. Korea had been split in half along the 38th parallel, with the USSR holding the north and the USA holding the south. As each sponsor state helped its occupied area to set up their own government, the two Koreas moved in increasingly different directions. Although negotiations for reunification continued almost up to the outbreak of war, tensions rose throughout the period especially from 1948 onwards.
On June 25th, 1950, North Korean forces poured over the border into South Korea, and the war began. The South Koreans were swiftly joined by a US-led coalition backed by the United Nations (the USSR was boycotting the UN Security Council at this point, and was thus unable to veto this action). The would last into 1953, costing hundreds of thousands of lives, until a ceasfire was negotiated, with the border still set roughly at the 38th parallel with little change to its pre-war location.
We Didn’t Start The Fire — Billy Joel