If America feels threatened it will attack. History has proven this time and time again. If any country launches a missile towards America they may as well prepare to meet their maker.
Any nation that observes our bickering with each other over Vietnam, Iraq, or Afghanistan will be making a major mistake to think we would not unify to defend (our own) shores. North Korea is sticking it's finger in the eye of giant in hopes of gaining some "respect".
Having said that if it is just another conflict between neighboring countries there will be debates about whether or not we stand by our "friends" or let them dangle in the wind. North Korea however is claiming they will be able to launch missiles towards America.