The cause of the Korean War in the 1950s was North Korea's invasion of South Korea. The war began on June 25, 1950, when North Korea, with the support of the Soviet Union and China, invaded South Korea. The invasion was part of North Korea's attempt to unify the Korean peninsula under communist rule. The United States and its allies, including South Korea and the United Nations, intervened in the conflict in support of South Korea, leading to a three-year war that ended in a stalemate and the establishment of a demilitarized zone between North and South Korea.