The Chinese intervention in late October 1950 bolstered the Korean People's Air Force (KPAF) of North Korea with the MiG-15, one of the world's most advanced jet fighters. The fast, heavily armed MiG outflew first-generation UN jets such as the F-80 (United States Air Force) and Gloster Meteors (Royal Australian Air Force), posing a real threat to B-29 Superfortress bombers even under fighter escort. Fearful of confronting the United States directly, the Soviet Union denied involvement of their personnel in anything other than an advisory role, but air combat quickly resulted in Soviet pilots dropping their code signals and speaking over the wireless in Russian. This known direct Soviet participation was a casus belli that the UN Command deliberately overlooked, lest the war for the Korean peninsula expand to include the Soviet Union, and potentially escalate into atomic warfare.
Who denied having anything more than an advisory role in the Korean War?
the Soviet Union