North Korean army was nearly totally decimated by American army before Chinese army entered the Korean war, small remnants of NK army was driven by US army to the mountains of Korean-Chinese border. The air cover provided by Soviet Union was not major, not even enough at times to secure the cover for Chinese army supply lines, SU air force only covered up to 300 km from Chinese border, the rest to the frontline supply line was the responsibility of young Chinese air force, further, SU air force didn't engage US air force until months after Chinese army entered the Korean war when the tide of the war was turned by Chinese army.