The Korean War was a military conflict between the South Korea and North Korea that started on June 25, 1950. Although armed hostilities ended with the Korean Armistice Agreement on July 27, 1953, the ceasefire was not a treaty under international law and so a technical state of war remains. The United States led the UN coalition supporting the South Korea, and the Soviet Union and Communist China supported North Korea. The Korean War was the first armed conflict and proxy war of the Cold War and escalated tensions between the West and the East. In September 1949, the Soviets tested their first atomic bomb, which made a limited war virtually impossible.