There are some stories circulating to the effect that NATO is going to do this or that if North Korea launches an attack, e.g. ‘NATO to protect US from N. Korea Attack’ here and ‘Senior NATO Official Will Not Rule Out Conflict with N. Korea’ here).
All the hard evidence so far indicates that NATO doesn’t know what it will do until an attack or something like it occurs. The North Atlantic Council would have to first meet to discuss the situation and options for any possible alliance response in the light of what just happened. Remember, Article 5 is about an armed attack on Allies in Europe or North America, not in Asia. An interview the NATO Secretary General gave to the Japanese media on 16 April during his visit to Japan makes it pretty clear:
The key point is around minute 1:
“If a NATO ally is attacked it will probably be raised for consultation among NATO allies. NATO allies will discuss the situation and make decisions based on the specific circumstances”
This is all the SecGen can say, which is no more than an explanation of the NATO process following the NATO Charter – specifically Article 4 (discussion), rather than 5 (collective action). And yet saying ‘we don’t know what we will do’ has more influence than saying nothing at all.
As for my earlier suggestions about employing NATO’s Article 4, Philip Cane at Atlantic Community has some practical suggestions about what NATO could do before an attack by the North (read at ‘South Korea:NATO’s Asian Pivot‘).