If there was any military strike against a nation under the NATO organization, then no matter what, the U.S. will retaliate as will any other nation against the threat.
A war will not start from just North Korea and their bombs, they know, if they start nuking the shit out of people right now, then they are screwed beyond all relief, and, be charged with all damages and be sued beyond all relief for possession of nuclear warfare and attempting to break the "Code of War".
Nuclear warfare wont be used unless someone uses it first. No one wants to use it first because no matter what, there's going to cause the whole world into a huge controversial debate whether or not either side is guilty or not.
That is the exact reason the U.S. is invading Afghanistan, in search of Osama bin Laden, and the reason we invaded Iraq. If the U.S. was to leave these nations without a proper guidance and properly established unitary system, that specific nation WILL convert to communism. This would be a repeat of the Cold War, the primary leaders trying to convert as much countries into their unit as possible. They know, if they can receive more support, a war can strike any minute. If the U.S. can bind the countries into a unified force that the opposing forces can trifle with, then there will be no threat. Putting the enemy into a submission.