Nope, it's not an invasion. Do you even know what an "invasion" is? The Republic of Korea had formally requested US military assistance and the two nations had already been allies from before the war. Hopefully your head is not so twisted that you can recognize the difference between an invasion versus a military assistance to help booster defensive efforts against an invasion. I must say, though, my hope there is not very high.

As for China, Chinese military received a assistance from the Chinese government and was a national army in all practical aspect. Are you going to deny that? Plus--and more importantly--the Chinese Army was helping an invader, as opposed to helping a defensive effort against an invasion that NK initiated. Rather a key distinction that you keep choosing to not address.