The Non-Aggression Principle (NAP) is the only criterion for libertarianism. One might expect libertarians to be contemplating deeply and writing the most scholarly articles on the topic. One might expect libertarians to be forming the most precise definition of aggression and non-aggression. Alas, a search of the most prominent libertarians yields only passing references on the topic. Libertarians believe that the NAP must be self-evident, despite the lack of agreement on the precise definition of what constitutes aggression and what does not.
This matters not only as an academic endeavor. More importantly, it is the libertarian position of non-intervention in foreign policy that depends on a clear understanding of what the principled libertarian view is on foreign aggression. Presidential candidate Ron Paul was perceived by many conservatives, who otherwise supported his libertarian positions, to be off putting for his foreign policy, not because they believed so much in US military intervention, but because the libertarian position of non-intervention is effectively silent on when it is appropriate to deploy military assets in the face of foreign aggression to defend American interests. A libertarian candidate for Commander-in-Chief cannot evade this essential topic, because it is the foremost qualification for the job. It is not enough to hand wave a general remark about supporting a strong national defense. Because of the libertarian position on non-intervention it is incumbent upon a libertarian candidate for POTUS to assure the citizens that with restraint also comes an impassioned intolerance for foreign aggression and no hesitation to deploy overwhelming military force and unspeakable violence in retaliation to any foreign power that initiates force against America. Non-intervention does not mean pacifism or appeasement or isolationism or weakness.
Now what exactly constitutes aggression? And what is the proper response to various acts of aggression? Silence on these questions is what is losing libertarians nominations, because a void in leadership on matters of defense is catastrophic to the preservation of liberty at home.