Russian and Chinese sources still disagree on what exactly Stalin promised Mao in order to secure Chinese intervention. The documents thus far available from the Russian archives indicate that Stalin never planned to use his MiG-15s and anti-aircraft forces for anything other than defending Chinese industry and supply lines. However, the Chinese claim that Stalin promised complete air support for their ground forces.
In any event, the Chinese army went into combat against U.S. and South Korean troops in late October 1950 without air cover or bomber support. This alleged betrayal by Stalin was a critical point in the eventual breakdown in Sino-Soviet relations.