North Korean invasion of South Korea, 1950
Korea 1949-53 | Cold War Documentary
CNN Cold War dokumentary [45 min] (In the textbook pages 76 - 83.)
The Korean War and its origins, 1945-53 | Original documents
Primary source: CIA's secret report
"OPINION ON LIKELIHOOD OF SOVIET OR CHINESE COMMUNIST ATTEMPTS TO SETTLE KOREAN WAR" (Dated on 9 June 1951)