After initially insisting on the peaceful reunification of Korea, Josif Stalin suddenly decided in early 1950 to give North Korean leader Kim Il Sung permission to invade South Korea. Documents from the Russian archives and materials published in China help explain this abrupt shift in Stalin’s position. They show that Stalin carefully assessed the likely American reaction and mistakenly concluded that North Korean forces would quickly seize South Korea, giving the United States no opportunity to respond. The documents also reveal that Stalin’s attitude toward Korea was strongly influenced by Sino-Soviet relations in 1949-1950, particularly his desire to maintain Soviet privileges on Chinese territory and his concern that Beijing would challenge Moscow’s leadership of the international Communist movement. Stalin believed that a North Korean invasion of the South would greatly strengthen the Soviet Union’s leverage vis-à-vis China.