I'd say that the fall of the Soviet Union discredited several ideas on the left and the right: on the left, the idea that the state should own most of the means of production; on the right, the idea of isolationism, or non-interventionism. It is now patently obvious that if the US had not drawn a proverbial line in the sand through Germany, the Soviets would now own large blocks of Western Europe that would be struggling in the same way that Eastern Europe now does.
Larison, of course, has a snappish rejoinder on behalf of non-interventionism. I would only say that even if Megan's right, this would better explain why libertarians backed the broad conception of a War on Terror than why they lined up to support the invasion of Iraq. If the end of the Cold War vindicated anything, surely, it was containment rather than "rollback," which was roughly the policy that the Bush Administration adopted vis-a-vis Saddam.
My own explanation would be that the character of the post-9/11 threat - an anti-modern, anti-liberal religious movement - dovetailed perfectly with the shifting character of American libertarianism, which with the decline of socialism and the rise of lifestyle politics was already increasingly inclined to view a resurgent religious conservatism, rather than Marxist-Leninist statism, as the greater threat to its worldview. This dovetailing, in turn, bred a distinctly un-libertarian zeal for a crusading foreign policy among people who otherwise wouldn't have bought into it. Just as Evelyn Waugh's traditionalist Catholic Guy Crouchback privately rejoiced at the Molotov-Ribbentrop pact, because it meant that "the enemy at last was plain in view, huge and hateful, all disguise cast off ... It was the Modern Age in arms," many libertarians instinctly leaped to interpret the 9/11 moment the way Andrew did - as the opening salvo in a grand "religious war," with secular modernity ranged on one side and every kind of "fundamentalism" on the other. Inevitably, it was Christopher Hitchens, a crypto-libertarian of sorts, who captured this spirit best:
... here was a direct, unmistakable confrontation between everything I loved and everything I hated. On one side, the ethics of the multicultural, the secular, the skeptical, and the cosmopolitan ... On the other, the arid monochrome of dull and vicious theocratic fascism. I am prepared for this war to go on for a very long time. I will never become tired of waging it, because it is a fight over essentials.
With such visions in the air, overreach was inevitable.