One must have something in mind when applying a label to oneself. A label subject to as many divergent associations as "conservative" makes this requirement harder to meet, but no less urgent.
My preferred interpretation stems from an article written by Lew Rockwell, that appeared in Liberty magazine some years ago. Rockwell was, at that time, setting forth on the ideological journey that would put him at odds with most other American libertarians and conservatives. Yet there were several observations in that article that ought to have been taken quite seriously by members of both camps -- indeed, by members of all American political designations. Central among them was this:
The key words of that assertion are the three at the end: preservation and defense. The target of such efforts, by Rockwell's lights, should be our civilization and culture as a whole and in fundamentals, rather than any detail excrescence. Thus, the top marginal tax rate, whatever it might be, is relatively unimportant, but the rule of law, a justice system blind to identity and group affiliation, free speech, free markets, and an ethic of public decency must be upheld at all costs.
To me, this expresses the core of conservatism. A conservative seeks to protect the fundamental principles from which the nation he loves has sprung. He's willing to entertain detail differences about specific policies with his fellows. He'll argue reasonably with anyone who's in accord with him on the pre-eminence of those fundamentals. But he'll brook no assault on them; that's tantamount to treason.
A conservative is a defender.
Here is where the Libertarian Party has committed its most grievous misstep. Party spokesmen and candidates have interpreted the assaults of Black Tuesday, and other Islamic terror attacks on Americans and their interests, as arguments for an American retreat from the Middle East and a return to a non-interventionist foreign policy. The milder ones argue that Operation Enduring Freedom and Operation Iraqi Freedom were at best terrible mistakes; others claim that the expeditions exceeded the federal government's Constitutional authority. The question of what degree of political and military engagement with other nations would be best is debatable, but to argue that America had no right to respond to a terrorist atrocity on its own soil, thousands killed and tens of billions of dollars in property destroyed, puts its proponent beyond the pale for anyone who regards our civilization as worthy of defense.
Of course, Buchananites have taken the same position. They receive more tolerance from mainstream conservatives because of their premise: that our military expeditions have actually harmed American interests, and our ability to defend them, over the long term. Even so, the intimation that the best thing to have done was not to respond sits very poorly with the overwhelming majority of conservatives, sufficiently so that Buchanan is now considered unwelcome in mainstream conservative gatherings.
In the precursor to this essay, Tom Van Dyke counterpoised "conservative" to "progressive." Well, yes and no. Wholesome conservatism will embrace changes that are consistent with the fundamentals it seeks to preserve and defend, if those changes can be shown to yield net positive results -- progress -- by some widely accepted standard. Progressivism, historically, has been dismissive of such constraints. Its progenitors were fond of saying that "reality is inherently unfinished," and that a sufficient application of will and effort can transform it into whatever we want it to be. Such a doctrine vitiates the very notion of an enduring principle.
A classical liberal who upholds a set of fundamental principles is also a conservative, whether or not he chooses to style himself as such.