It's the army of UN, not US. Plus the UN army is legal and authorized by UN. Further please remember that, at that time Soviet was one of the 5 安理会常任理事国 and it's capable to cancel any UN action by it's voting power. But it didn't, why? Do you think over it?
How did you obtain that information? From the publication in China?
Please do not make comments like this. Respect others and yourself. Thank you.