NATO - Official text: The North Atlantic Treaty, 04-Apr.-1949
looking at the charter quickly (first time in a while) i would think that the action would be legitimated via article 3 and 5, which introduce the category "security" as a justification for acting, and the repeated references to co-ordination with the un that run throughout. i don't see it as a problem from that viewpoint.
my ambivalences have much more to do with the kind of objections tariq ali outlined in the guardian yesterday---the hijacking of a revolution, the selective application of this "ethical" argument for justifying intervention, etc.. even as i think that there had to be some kind of intervention in libya to prevent a massacre (which still might happen)....