I don't think you, or majority of the people, fully appreciate the might of USA. They're insanely powerful and even the likes of China, with all its economy and military might, is NOTHING in front of USA. Americans don't really fear anyone (except Israel) and could easily take on 75% of the world simultaneously and still emerge victorious.
If USA decides to lighten up Iranian skies (I hope NOT), there is F All China will do, other than to issue a very strongly worded statement. No one in the western world takes China's words seriously.
Let's not talk about the other 2 nuclear 'powers'.