At 8:45 a.m., Sept. 11, 2001, we were living in the post-Cold War era. At 9:37 a.m., just 52 minutes later, as the third hijacked airliner careened into the Pentagon, the post–9/11 era had begun. Everyone told us that everything had changed.

It was the beginning of a new chapter in history. The image of thousands of people perishing as the Twin Towers collapsed in a cascade of fire and dust, live on television, was a bookmark for the ages. There was a world before this tragedy, and then there was something very different that was about to follow. It is tempting to assume that this attitude was just another example of American narcissism. (The United States was attacked, so the world had changed.) But that wasn’t the case. A poll taken shortly after the attacks by the Pew Research Center found a remarkable degree of agreement among opinion leaders around the world about what the September 11 attacks represented. In Western Europe, 76 percent of those polled said the events of that day had amounted to a turning point in world history. In Russia and Asia, 73 and 69 percent of people agreed. In the Middle East and Latin America, the percentage of opinion leaders who believed 9/11 marked the beginning of a new era rose to 90 percent. Rarely have so many agreed about the meaning of a single moment.

Five years on, this response must be understood as one being born out of shock. Certainly, for some, there could not have been a more life-changing moment. Collectively, we feared what was about to end. Globalization would surely grind to a halt. Borders—in particular, the need to maintain them—would undergo a renaissance as governments looked to shield themselves from the next attack. Global trade, capital flows, and immigration could no longer be what they once were. National economies would cool, as the realization of a...