[Zythus]

The thesis of Devonin's post was to show how self sufficent and powerful USA is. How powerful? even the U.N. could not stand up to their power. Hence, it is frankly absurd to suggest that another country would even dare to assault the US. First of all, they face total destruction. Secondly, if US doesn't like a country, chances are that their hatred would induce the minds of other countries to hate them as well.
Since you really can't cope with long and informative posts, heres it simple:

Why another country cannot missile the US:
-US army will destroy them. Period.
-US will get pissed, and if US is pissed, other countries would follow.

Heres an answer to the next possible question of yours:

Why would other countries follow?
Because US is the strongest power and their influence causes other countries to side with them, Against the missiling country.