By 1949, South Korean and US military actions had reduced the active number of indigenous communist guerrillas in the South from 5,000 to 1,000. However, Kim Il-sung believed that widespread uprisings had weakened the South Korean military and that a North Korean invasion would be welcomed by much of the South Korean population. Kim began seeking Stalin's support for an invasion in March 1949, traveling to Moscow to attempt to persuade him. Stalin initially did not think the time was right for a war in Korea. PLA forces were still embroiled in the Chinese Civil War, while US forces remained stationed in South Korea.

By spring 1950, he believed that the strategic situation had changed: PLA forces under Mao Zedong had secured final victory in China, US forces had withdrawn from Korea, and the Soviets had detonated their first nuclear bomb, breaking the US atomic monopoly. As the US had not directly intervened to stop the communist victory in China, Stalin calculated that they would be even less willing to fight in Korea, which had much less strategic significance. The Soviets had also cracked the codes used by the US to communicate with their embassy in Moscow, and reading these dispatches convinced Stalin that Korea did not have the importance to the US that would warrant a nuclear confrontation. Stalin began a more aggressive strategy in Asia based on these developments, including promising economic and military aid to China through the Sino-Soviet Treaty of Friendship, Alliance, and Mutual Assistance. In April 1950, Stalin gave Kim permission to attack the government in the South under the condition that Mao would agree to send reinforcements if needed. For Kim, this was the fulfillment of his goal to unite Korea after its division by foreign powers. Stalin made it clear that Soviet forces would not openly engage in combat, to avoid a direct war with the US.

Kim met with Mao in May 1950. Mao was concerned the US would intervene but agreed to support the North Korean invasion. China desperately needed the economic and military aid promised by the Soviets. However, Mao sent more ethnic Korean PLA veterans to Korea and promised to move an army closer to the Korean border. Once Mao's commitment was secured, preparations for war accelerated.