Maybe most tellingly, the GOP has backed far, far away from the national-security policy of the Bush years. Denunciations of the National Security Agency’s surveillance programs have become standard-issue Republican rhetoric. The tally of likely votes just before President Obama withdrew his request for authority to strike Syria last September showed 170 definite or likely Republican “no” votes in the House of Representatives.

Despite the self-flattering claims of libertarians, the Republicans' post-2009 libertarian turn is not a response to voter demand. The areas where the voting public has moved furthest and fastest in a libertarian direction—gay rights, for example—have been the areas where Republicans have moved slowest and most reluctantly. The areas where the voting public most resists libertarian ideas—such as social benefits—are precisely the areas where the GOP has swung furthest and fastest in a libertarian direction.

Nor is it the strength and truth of libertarian ideas that explains their current vogue within the Republican Party. Libertarians have been most influential inside the GOP precisely where they have been—and continue to be—most blatantly wrong, such as when they predicted that the cheap money policies of the Federal Reserve would incite hyperinflation or that the United States teetered on the precipice of a debt crisis.

Much of the libertarian appeal is probably as simple as the isolationist reaction that tends to overtake the United States after military conflicts. Libertarians always want to cut the defense budget, and after Iraq and Afghanistan, many orthodox Republicans felt in the mood to agree with them. Yet it’s also true that post-Iraq and post-Afghanistan, the Republicans remain the party of assertive nationalism—and the party more comfortable with the use of force. It’s telling that prior to running for president, Rand Paul has reinvented his own past views both on Israel and on drone attacks inside Afghanistan. Isolationism alone doesn't explain the rise of libertarianism inside the GOP.

Libertarianism diverges from ordinary conservatism in many ways, but perhaps most fundamentally in this: Whereas ordinary conservatism emphasizes the inefficiency and ineffectiveness of government action, libertarianism presents government as alien and malign. When Rand Paul rose early in 2013 to deliver the longest talking filibuster since Strom Thurmond battled civil rights in 1957, he did so not to oppose some new bureaucracy or tax. No, Paul rose to denounce the supposedly looming danger of lethal drone attacks on ordinary law-abiding Americans. “I will speak today until the President responds and says no, we won't kill Americans in cafes; no, we won't kill you at home in your bed at night; no, we won't drop bombs on restaurants,” he said. Repeatedly, Paul insisted that he was not accusing President Obama of plotting the murder of American citizens. Equally repeatedly, however, he made clear that he considered the danger of presidential murder of people like himself real and imminent.