To claim that it is not enough for the libertarian to oppose aggression is to fall into the trap that destroyed classical liberalism the first time, and transformed it into modern liberalism. How, after all, did the classical liberalism of the eighteenth and nineteenth centuries become the state-obsessed liberalism of the twentieth and twenty-first centuries? How did the once-venerable word liberalism become perverted in the first place? Precisely because of thickism. Sure, twentieth-century liberals said, we favor liberty, but since mere negative liberty — that is, restrictions on the state — doesn’t appear to yield a sufficiently egalitarian result, we need more than that. In addition to restrictions on some state activity, we need the expansion of other forms of state activity.
All of these additional claims are a distraction from the central principle: if you oppose the initiation of physical force, you are a libertarian. Period. Now how hard was that?