And after all these results you dare to say, that US have the most powerfull army??? When even it's own citizens don't want to fight for their own motherland??? I'm shocked really. Ask every Russian, or Chinesse about same topic - would they fight if their motherland is invaded - i think 99,99% would say - "yes". And we did it for about 300 years.
Morale... All is about morale of troops. Army can't fight even with best weapons, if they are used by cowards...
And about invasion. It's not so unrealistic... Geographical isolation - is not a problem. China can invade America through Canada and Alaska, and with modern transporting techs - it's much more easier than you may think...
Air Force... Navy...
Major role in full scale war play infantry. Yes, typical man or women, with one head, two arms, two legs and with assault rifle. Support lines for Aircraft (and Airports) can be bombed, fuel reserves - sabotaged, SAM's are more cheaper in use and support that an aircraft costing 6,000,000$.
And about nukes... No one will bomb their own territory... And I think, not invaders nor America would use nukes at all. Everyone want to live. America would more likely to surrender, but it would not use nukes. You see, no one, not so-called president, not a general or officers in nuklear silos would launch nuklear war. Morale...