Historians still debate the relative positions of the USSR and Communist China re: involvement in the Korean War.
The traditional view has been that, emerging from the catastrophe of WWII, Joseph Stalin insisted that the North Korean government exhaust all efforts to bring a peaceful solution to the North-South divide prior to initiating war.
However in late 1949, Stalin apparently approved Kim Il-sung’s plan to attack South Korea, but cautioned that the USSR would not participate beyond supplying North Korea with weapons. Stalin urged the North Korean leader to approach Mao Zedong for any further assistance he needed.
After the prolonged Sino-Japanese and Chinese Civil wars, Mao Zedong may not have been enthusiastic about yet another war in the Eastern Pacific. Nevertheless, his motivation is likewise controversial.
One view, commonly held in the West, is that when the United States landed at Inchon, Mao recognized that the United States and the UN could quickly overrun North Korea and threaten China’s border. At that point, Mao decided to support North Korea with or without Soviet aid.
Another view, is that Mao’s overriding criteria was not whether US forces would cross the 38th parallel, but whether their dispatch by Chinese forces might help to solidify the new Chinese Communist regime.
Regardless of its motivation, In September 1950 China mobilized troops and resources in preparation for possible escalation, and issued a warning through Indian ambassador Kavalam Madhava Panikkar that China would intervene in the war if the American troops entered North Korea.
The US National Security Council and Secretary of State disregarded these warnings, telling the press that Chinese intervention would be “sheer madness.”
After the U.S. troops crossed the 38th parallel on September 25, China made the decision to intervene.
On October 14, 1950 the Thirteenth Corps of China’s People’s Volunteer Army marched across the Yalu River marking the border between China and North Korea as the first of hundreds of thousands of Chinese troops to invade the Korean Peninsula.