This study aims to analyze the United Nation's intelligence failure to predict the invasion of South Korea by the North Korean army at the beginning of the Korean War. The study will analyze the intelligence failures by reviewing US intelligence activities and estimates from 1949, when Kim Il-Sung planned the invasion of South Korea to June 25, 1950, when the attacks began. The Korean War was the first and the largest military conflict during the Cold War, and the most horrible. Even though the US's intelligence agencies such as CIA, the Far East Command (FEC), and the Embassy in Seoul provided some information on the movement of North Korean forces, no early warnings of a North Korean invasion made it to Truman before the Korean War, which resulted in an intelligence failure. This study argues the reasons for the US's intelligence failures were; cognitive failure in underestimating the strategic value of the Korean peninsula, setting a lower priority for the Korean peninsulas compared to the Soviet Union and Taiwan in intelligence collecting. In addition, judgment failures resulted from strategic deceptions and subterfuge by the Soviet Union and the North Korean army, and the consequence of alert fatigue from the “cry wolf effect” of the South Korean government.