The Korean War started after North Korea led by their leader, Kim Il Sung invaded South Korea in the month of June 1950. Kim Il Sung wanted to unite North and South Korea under his communist rule. The US got involved after the United Nations Security Council condemned the invasion on 30th June 1950.
I agree with kibe. The North Koreans crossed the 38th parallel to invade the South. It was Gen. Douglas McArthur who was called upon to stop/ repulse the invasion with the 1st test of United Nations collective military cooperation.